High speed media access control

ABSTRACT

Embodiments disclosed herein for MAC processing for efficient use of high throughput systems and that may be backward compatible with various types of legacy systems. In one aspect, a data transmission structure comprises a consolidated poll and one or more frames transmitted in accordance with the consolidated poll. In another aspect, a Time Division Duplexing (TDD) data transmission structure comprises a pilot, a consolidated poll, and zero or more access point to remote station frames in accordance with the consolidated poll. In one aspect, frames are transmitted sequentially with no or substantially reduced interframe spacing. In another aspect, a guard interframe spacing may be introduced between frames transmitted from different sources, or with substantially different power levels. In another aspect, a single preamble is transmitted in association with one or more frames. In another aspect, a block acknowledgement is transmitted subsequent to the transmission of one or more sequential frames.

PRIORITY CLAIMS

The present application for patent is a Continuation of patentapplication Ser. No. 10/964,321 entitled HIGH SPEED MEDIA ACCESS CONTROLfiled Oct. 13, 2004, which claims priority to the followingapplications:

Provisional Application No. 60/511,750 entitled “Method and Apparatusfor Providing Interoperability and Backward Compatibility in WirelessCommunication Systems” filed Oct. 15, 2003;

Provisional Application No. 60/511,904 entitled “Method, Apparatus, andSystem for Medium Access Control in a High Performance Wireless LANEnvironment” filed Oct. 15, 2003;

Provisional Application No. 60/513,239 entitled “Peer-to-PeerConnections in MIMO WLAN System” filed Oct. 21, 2003;

Provisional Application No. 60/526,347 entitled “Method, Apparatus, andSystem for Sub-Network Protocol Stack for Very High Speed Wireless LAN”filed Dec. 1, 2003;

Provisional Application No. 60/526,356 entitled “Method, Apparatus, andSystem for Multiplexing Protocol data Units in a High PerformanceWireless LAN Environment” filed Dec. 1, 2003;

Provisional Application No. 60/532,791 entitled “Wireless CommunicationsMedium Access Control (MAC) Enhancements” filed Dec. 23, 2003;

Provisional Application No. 60/545,963 entitled “Adaptive CoordinationFunction (ACF)” filed Feb. 18, 2004;

Provisional Application No. 60/576,545 entitled “Method and Apparatusfor Robust Wireless Network” filed Jun. 2, 2004;

Provisional Application No. 60/586,841 entitled “Method and Apparatusfor Distribution Communication Resources Among Multiple Users” filedJul. 8, 2004; and

Provisional Application No. 60/600,960 entitled “Method, Apparatus, andSystem for Wireless Communications” filed Aug. 11, 2004.

All of the applications listed above are assigned to the assignee hereofand hereby expressly incorporated by reference herein.

BACKGROUND

1. Field

The present invention relates generally to communications, and morespecifically to medium access control.

2. Background

Wireless communication systems are widely deployed to provide varioustypes of communication such as voice and data. A typical wireless datasystem, or network, provides multiple users access to one or more sharedresources. A system may use a variety of multiple access techniques suchas Frequency Division Multiplexing (FDM), Time Division Multiplexing(TDM), Code Division Multiplexing (CDM), and others.

Example wireless networks include cellular-based data systems. Thefollowing are several such examples: (1) the “TIA/EIA-95-B MobileStation-Base Station Compatibility Standard for Dual-Mode WidebandSpread Spectrum Cellular System” (the IS-95 standard), (2) the standardoffered by a consortium named “3rd Generation Partnership Project”(3GPP) and embodied in a set of documents including Document Nos. 3G TS25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214 (the W-CDMAstandard), (3) the standard offered by a consortium named “3rdGeneration Partnership Project 2” (3GPP2) and embodied in “TR-45.5Physical Layer Standard for cdma2000 Spread Spectrum Systems” (theIS-2000 standard), and (4) the high data rate (HDR) system that conformsto the TIA/EIA/IS-856 standard (the IS-856 standard).

Other examples of wireless systems include Wireless Local Area Networks(WLANs) such as the IEEE 802.11 standards (i.e. 802.11(a), (b), or (g)).Improvements over these networks may be achieved in deploying a MultipleInput Multiple Output (MIMO) WLAN comprising Orthogonal FrequencyDivision Multiplexing (OFDM) modulation techniques. IEEE 802.11(e) hasbeen introduced to improve upon some of the shortcomings of previous802.11 standards.

As wireless system designs have advanced, higher data rates have becomeavailable. Higher data rates have opened up the possibility of advancedapplications, among which are voice, video, fast data transfer, andvarious other applications. However, various applications may havediffering requirements for their respective data transfer. Many types ofdata may have latency and throughput requirements, or need some Qualityof Service (QoS) guarantee. Without resource management, the capacity ofa system may be reduced, and the system may not operate efficiently.

Medium Access Control (MAC) protocols are commonly used to allocate ashared communication resource between a number of users. MAC protocolscommonly interface higher layers to the physical layer used to transmitand receive data. To benefit from an increase in data rates, a MACprotocol must be designed to utilize the shared resource efficiently. Itis also generally desirable to maintain interoperability with alternateor legacy communication standards. There is therefore a need in the artfor MAC processing for efficient use of high throughput systems. Thereis a further need in the art for such MAC processing that is backwardcompatible with various types of legacy systems.

SUMMARY

Embodiments disclosed herein address the need for MAC processing forefficient use of high throughput systems and that may be backwardcompatible with various types of legacy systems. In one aspect, a datatransmission structure comprises a consolidated poll and one or moreframes transmitted in accordance with the consolidated poll. In anotheraspect, a Time Division Duplexing (TDD) data transmission structurecomprises a pilot, a consolidated poll, and zero or more access point toremote station frames in accordance with the consolidated poll.

In one aspect, frames are transmitted sequentially with no orsubstantially reduced interframe spacing. In another aspect, a guardinterframe spacing may be introduced between frames transmitted fromdifferent sources, or with substantially different power levels. Inanother aspect, a single preamble is transmitted in association with oneor more frames. In another aspect, a block acknowledgement istransmitted subsequent to the transmission of one or more sequentialframes. In another aspect, a consolidated poll is transmitted, and oneor more frames are transmitted in association therewith. Various otheraspects are also presented.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an example embodiment of a system including a high-speed WLAN;

FIG. 2 depicts an example embodiment of a wireless communication device,which may be configured as an access point or user terminal;

FIG. 3 depicts 802.11 interframe spacing parameters;

FIG. 4 depicts an example physical layer (PHY) transmission segmentillustrating the use of DIFS plus backoff for access according to theDCF;

FIG. 5 depicts an example physical layer (PHY) transmission segmentillustrating the use of SIFS before an ACK, with higher priority than aDIFS access;

FIG. 6 illustrates segmenting large packets into smaller fragments withassociated SIFS;

FIG. 7 depicts an example physical layer (PHY) transmission segmentillustrating a TXOP with per-frame acknowledgment;

FIG. 8 illustrates a TXOP with block acknowledgment;

FIG. 9 depicts an example physical layer (PHY) transmission segmentillustrating a polled TXOP using HCCA;

FIG. 10 is an example embodiment of a TXOP including multipleconsecutive transmissions without any gaps;

FIG. 11 depicts an example embodiment of a TXOP illustrating reducingthe amount of preamble transmission required;

FIG. 12 depicts an example embodiment of a method for incorporatingvarious aspects, including consolidating preambles, removing gaps suchas SIFS, and inserting GIFs as appropriate;

FIG. 13 depicts an example physical layer (PHY) transmission segmentillustrating consolidated polls and their respective TXOPs;

FIG. 14 depicts an example embodiment of a method for consolidatingpolls;

FIG. 15 illustrates an example MAC frame;

FIG. 16 illustrates an example MAC PDU;

FIG. 17 depicts an example peer-to-peer communication;

FIG. 18 depicts a prior art physical layer burst;

FIG. 19 depicts an example physical layer burst, which may be deployedfor peer-peer transmission;

FIG. 20 depicts an example embodiment of a MAC frame including anoptional ad hoc segment;

FIG. 21 depicts an example physical layer burst;

FIG. 22 depicts an example method for peer-peer data transmission;

FIG. 23 depicts an example method for peer-peer communication;

FIG. 24 depicts an example method for providing rate feedback for use inpeer-peer connection;

FIG. 25 illustrates managed peer-peer connection between two stationsand an access point;

FIG. 26 illustrates a contention based (or ad hoc) peer-peer connection;

FIG. 27 depicts an example MAC frame illustrating managed peer-peercommunication between stations;

FIG. 28 illustrates supporting both legacy and new class stations on thesame frequency assignment;

FIG. 29 illustrates the combination of legacy and new class media accesscontrol;

FIG. 30 depicts an example method for earning a transmit opportunity;

FIG. 31 depicts an example method for sharing a single FA with multipleBSSs;

FIG. 32 illustrates overlapping BSSs using a single FA;

FIG. 33 depicts an example method for performing high-speed peer-peercommunication while interoperating with a legacy BSS;

FIG. 34 illustrates peer-peer communication using MIMO techniques bycontending for access on a legacy BSS;

FIG. 35 depicts encapsulation of one or more MAC frames (or fragments)within an aggregated frame;

FIG. 36 depicts a legacy MAC frame;

FIG. 37 illustrates an example uncompressed frame;

FIG. 38 illustrates an example compressed frame;

FIG. 39 illustrates another example compressed frame;

FIG. 40 illustrates an example Aggregation Header;

FIG. 41 illustrates an example embodiment of a Scheduled Access PeriodFrame (SCAP) for use in the ACF;

FIG. 42 illustrates how the SCAP may be used in conjunction with HCCAand EDCA;

FIG. 43 illustrates Beacon intervals comprising a number of SCAPsinterspersed with contention-based access periods;

FIG. 44 illustrates low-latency operation with a large number of MIMOSTAs;

FIG. 45 illustrates an example SCHED message;

FIG. 46 depicts an example Power Management field;

FIG. 47 depicts an example MAP field;

FIG. 48 illustrates example SCHED control frames for TXOP assignment;

FIG. 49 depicts a legacy 802.11 PPDU;

FIG. 50 depicts an example MIMO PPDU format for data transmissions;

FIG. 51 depicts an example SCHED PPDU;

FIG. 52 depicts an example FRACH PPDU; and

FIG. 53 illustrates an alternative embodiment of a method ofinteroperability with legacy systems.

DETAILED DESCRIPTION

Example embodiments are disclosed herein that support highly efficientoperation in conjunction with very high bit rate physical layers for awireless LAN (or similar applications that use newly emergingtransmission technologies). The example WLAN supports bit rates inexcess of 100 Mbps (million bits per second) in bandwidths of 20 MHz.

Various example embodiments preserve the simplicity and robustness ofthe distributed coordination operation of legacy WLAN systems, examplesof which are found in 802.11(a-e). The advantages of the variousembodiments may be achieved while maintaining backward compatibilitywith such legacy systems. (Note that, in the description below, 802.11systems are described as example legacy systems. Those of skill in theart will recognize that the improvements are also compatible withalternate systems and standards.)

An example WLAN may comprise a sub-network protocol stack. Thesub-network protocol stack may support high data rate, high bandwidthphysical layer transport mechanisms in general, including, but notlimited to, those based on OFDM modulation, single carrier modulationtechniques, systems using multiple transmit and multiple receiveantennas (Multiple Input Multiple Output (MIMO) systems, includingMultiple Input Single Output (MISO) systems) for very high bandwidthefficiency operation, systems using multiple transmit and receiveantennas in conjunction with spatial multiplexing techniques to transmitdata to or from multiple user terminals during the same time interval,and systems using code division multiple access (CDMA) techniques toallow transmissions for multiple users simultaneously. Alternateexamples include Single Input Multiple Output (SIMO) and Single InputSingle Output (SISO) systems.

One or more exemplary embodiments described herein are set forth in thecontext of a wireless data communication system. While use within thiscontext is advantageous, different embodiments of the invention may beincorporated in different environments or configurations. In general,the various systems described herein may be formed usingsoftware-controlled processors, integrated circuits, or discrete logic.The data, instructions, commands, information, signals, symbols, andchips that may be referenced throughout the application areadvantageously represented by voltages, currents, electromagnetic waves,magnetic fields or particles, optical fields or particles, or acombination thereof. In addition, the blocks shown in each block diagrammay represent hardware or method steps. Method steps can be interchangedwithout departing from the scope of the present invention. The word“exemplary” is used herein to mean “serving as an example, instance, orillustration.” Any embodiment described herein as “exemplary” is notnecessarily to be construed as preferred or advantageous over otherembodiments.

FIG. 1 is an example embodiment of system 100, comprising an AccessPoint (AP) 104 connected to one or more User Terminals (UTs) 106A-N. Inaccordance with 802.11 terminology, in this document the AP and the UTsare also referred to as stations or STAs. The AP and the UTs communicatevia Wireless Local Area Network (WLAN) 120. In the example embodiment,WLAN 120 is a high speed MIMO OFDM system. However, WLAN 120 may be anywireless LAN. Access point 104 communicates with any number of externaldevices or processes via network 102. Network 102 may be the Internet,an intranet, or any other wired, wireless, or optical network.Connection 110 carries the physical layer signals from the network tothe access point 104. Devices or processes may be connected to network102 or as UTs (or via connections therewith) on WLAN 120. Examples ofdevices that may be connected to either network 102 or WLAN 120 includephones, Personal Digital Assistants (PDAs), computers of various types(laptops, personal computers, workstations, terminals of any type),video devices such as cameras, camcorders, webcams, and virtually anyother type of data device. Processes may include voice, video, datacommunications, etc. Various data streams may have varying transmissionrequirements, which may be accommodated by using varying Quality ofService (QoS) techniques.

System 100 may be deployed with a centralized AP 104. All UTs 106communicate with the AP in one example embodiment. In an alternateembodiment, direct peer-to-peer communication between two UTs may beaccommodated, with modifications to the system, as will be apparent tothose of skill in the art, examples of which are illustrated below.Access may be managed by an AP, or ad hoc (i.e. contention based), asdetailed below.

In one embodiment, AP 104 provides Ethernet adaptation. In this case, anIP router may be deployed in addition to the AP to provide connection tonetwork 102 (details not shown). Ethernet frames may be transferredbetween the router and the UTs 106 over the WLAN sub-network (detailedbelow). Ethernet adaptation and connectivity are well known in the art.

In an alternate embodiment, the AP 104 provides IP Adaptation. In thiscase, the AP acts as a gateway router for the set of connected UTs(details not shown). In this case, IP datagrams may be routed by the AP104 to and from the UTs 106. IP adaptation and connectivity are wellknown in the art.

FIG. 2 depicts an example embodiment of a wireless communication device,which may be configured as an access point 104 or user terminal 106. Anaccess point 104 configuration is shown in FIG. 2. Transceiver 210receives and transmits on connection 110 according to the physical layerrequirements of network 102. Data from or to devices or applicationsconnected to network 102 are delivered to MAC processor 220. These dataare referred to herein as flows 260. Flows may have differentcharacteristics and may require different processing based on the typeof application associated with the flow. For example, video or voice maybe characterized as low-latency flows (video generally having higherthroughput requirements than voice). Many data applications are lesssensitive to latency, but may have higher data integrity requirements(i.e., voice may be tolerant of some packet loss, file transfer isgenerally intolerant of packet loss).

MAC processor 220 receives flows 260 and processes them for transmissionon the physical layer. MAC processor 220 also receives physical layerdata and processes the data to form packets for outgoing flows 260.Internal control and signaling is also communicated between the AP andthe UTs. MAC Protocol Data Units (MAC PDUs), also referred to asPhysical layer (PHY) Protocol Data Units (PPDUs), or frames (in 802.11parlance) are delivered to and received from wireless LAN transceiver240 on connection 270. Example techniques for conversion from flows andcommands to MAC PDUs, and vice versa, are detailed below. Alternateembodiments may employ any conversion technique. Feedback 280corresponding to the various MAC IDs may be returned from the physicallayer (PHY) 240 to MAC processor 220 for various purposes. Feedback 280may comprise any physical layer information, including supportable ratesfor channels (including multicast as well as unicast channels),modulation format, and various other parameters.

In an example embodiment, the Adaptation layer (ADAP) and Data LinkControl layer (DLC) are performed in MAC processor 220. The physicallayer (PHY) is performed on wireless LAN transceiver 240. Those of skillin the art will recognize that the segmentation of the various functionsmay be made in any of a variety of configurations. MAC processor 220 mayperform some or all of the processing for the physical layer. A wirelessLAN transceiver may include a processor for performing MAC processing,or subparts thereof. Any number of processors, special purpose hardware,or combination thereof may be deployed.

MAC processor 220 may be a general-purpose microprocessor, a digitalsignal processor (DSP), or a special-purpose processor. MAC processor220 may be connected with special-purpose hardware to assist in varioustasks (details not shown). Various applications may be run on externallyconnected processors, such as an externally connected computer or over anetwork connection, may run on an additional processor within accesspoint 104 (not shown), or may run on MAC processor 220 itself. MACprocessor 220 is shown connected with memory 255, which may be used forstoring data as well as instructions for performing the variousprocedures and methods described herein. Those of skill in the art willrecognize that memory 255 may be comprised of one or more memorycomponents of various types, that may be embedded in whole or in partwithin MAC processor 220.

In addition to storing instructions and data for performing functionsdescribed herein, memory 255 may also be used for storing dataassociated with various queues.

Wireless LAN transceiver 240 may be any type of transceiver. In anexample embodiment, wireless LAN transceiver 240 is an OFDM transceiver,which may be operated with a MIMO or MISO interface. OFDM, MIMO, andMISO are known to those of skill in the art. Various example OFDM, MIMOand MISO transceivers are detailed in co-pending U.S. patent applicationSer. No. 10/650,295, entitled “FREQUENCY-INDEPENDENT SPATIAL-PROCESSINGFOR WIDEBAND MISO AND MIMO SYSTEMS”, filed Aug. 27, 2003, assigned tothe assignee of the present invention. Alternate embodiments may includeSIMO or SISO systems.

Wireless LAN transceiver 240 is shown connected with antennas 250 A-N.Any number of antennas may be supported in various embodiments. Antennas250 may be used to transmit and receive on WLAN 120.

Wireless LAN transceiver 240 may comprise a spatial processor connectedto each of the one or more antennas 250. The spatial processor mayprocess the data for transmission independently for each antenna orjointly process the received signals on all antennas. Examples of theindependent processing may be based on channel estimates, feedback fromthe UT, channel inversion, or a variety of other techniques known in theart. The processing is performed using any of a variety of spatialprocessing techniques. Various transceivers of this type may use beamforming, beam steering, eigen-steering, or other spatial techniques toincrease throughput to and from a given user terminal. In an exampleembodiment, in which OFDM symbols are transmitted, the spatial processormay comprise sub-spatial processors for processing each of the OFDMsubchannels, or bins.

In an example system, the AP may have N antennas, and an example UT mayhave M antennas. There are thus M×N paths between the antennas of the APand the UT. A variety of spatial techniques for improving throughputusing these multiple paths are known in the art. In a Space TimeTransmit Diversity (STTD) system (also referred to herein as“diversity”), transmission data is formatted and encoded and sent acrossall the antennas as a single stream of data. With M transmit antennasand N receive antennas there may be MIN (M, N) independent channels thatmay be formed. Spatial multiplexing exploits these independent paths andmay transmit different data on each of the independent paths, toincrease the transmission rate.

Various techniques are known for learning or adapting to thecharacteristics of the channel between the AP and a UT. Unique pilotsmay be transmitted from each transmit antenna. The pilots are receivedat each receive antenna and measured. Channel state information feedbackmay then be returned to the transmitting device for use in transmission.Eigen decomposition of the measured channel matrix may be performed todetermine the channel eigenmodes. An alternate technique, to avoid eigendecomposition of the channel matrix at the receiver, is to useeigen-steering of the pilot and data to simplify spatial processing atthe receiver.

Thus, depending on the current channel conditions, varying data ratesmay be available for transmission to various user terminals throughoutthe system. In particular, the specific link between the AP and each UTmay be higher performance than a multicast or broadcast link that may beshared from the AP to more than one UT. Examples of this are detailedfurther below. The wireless LAN transceiver 240 may determine thesupportable rate based on whichever spatial processing is being used forthe physical link between the AP and the UT. This information may be fedback on connection 280 for use in MAC processing.

The number of antennas may be deployed depending on the UT's data needsas well as size and form factor. For example, a high definition videodisplay may comprise, for example, four antennas, due to its highbandwidth requirements, while a PDA may be satisfied with two. Anexample access point may have four antennas.

A user terminal 106 may be deployed in similar fashion to the accesspoint 104 depicted in FIG. 2. Rather than having flows 260 connect witha LAN transceiver (although a UT may include such a transceiver, eitherwired or wireless), flows 260 are generally received from or deliveredto one or more applications or processes operating on the UT or a deviceconnected therewith. The higher levels connected to either AP 104 or UT106 may be of any type. Layers described herein are illustrative only.

Legacy 802.11 MAC

As mentioned above, various embodiments detailed herein may be deployedso as to be compatible with legacy systems. The IEEE 802.11(e) featureset (which is turn backward compatible with earlier 802.11 standards),includes various features that will be summarized in this section, alongwith features introduced in earlier standards. For a detaileddescription of these functions, refer to the respective IEEE 802.11standard.

The basic 802.11 MAC consists of a Carrier Sense MultipleAccess/Collision Avoidance (CSMA/CA) based Distributed CoordinationFunction (DCF) and a Point Coordination Function (PCF). The DCF allowsfor access of the medium without central control. The PCF is deployed atan AP to provide central control. The DCF and PCF utilize various gapsbetween consecutive transmissions to avoid collisions. Transmissions arereferred to as frames, and a gap between frames is referred to as anInterframe Spacing (IFS). Frames may be user data frames, control framesor management frames.

Interframe spacing time durations vary depending on the type of gapinserted. FIG. 3 depicts 802.11 interframe spacing parameters: a ShortInterframe Spacing (SIFS), a Point Interframe Spacing (PIFS), and a DCFInterframe Spacing (DIFS). Note that SIFS<PIFS<DIFS. Thus, atransmission following a shorter time duration will have a higherpriority than one which must wait longer before attempting to access thechannel.

According to the carrier sense (CSMA) feature of CSMA/CA, a station(STA) may gain access to the channel after sensing the channel to beidle for at least a DIFS duration. (As used herein, the term STA mayrefer to any station accessing a WLAN, and may include access points aswell as user terminals). To avoid collision, each STA waits a randomlyselected backoff in addition to DIFS before accessing the channel. STAswith a longer backoff will notice when a higher priority STA beginstransmitting on the channel, and will thus avoid colliding with thatSTA. (Each waiting STA may reduce its respective backoff by the amountof time it waited before sensing an alternate transmission on thechannel, thus maintaining its relative priority.) Thus, following thecollision avoidance (CA) feature of the protocol, the STA backs-off arandom period of time between [0, CW] where CW is initially chosen to beCWmin, but increases by a factor of two at every collision, until amaximum value of CWmax.

FIG. 4 depicts example physical layer (PHY) transmission segment 400,which illustrates the use of DIFS plus backoff for access according tothe DCF. An existing transmission 410 utilizes the channel. Whentransmission 410 terminates, in this example, no higher priorityaccesses occur, and so new transmission 420 begins after DIFS and theassociated backoff period. In the discussion below, the STA makingtransmission 420 is said to have earned this opportunity to transmit, inthis case through contention.

SIFS is used during a frame sequence in which only a specific STA isexpected to respond to the current transmission. For example, when anAcknowledgement (ACK) is transmitted in response to a received frame ofdata, that ACK may be transmitted immediately following the receiveddata plus SIFS. Other transmission sequences may also use SIFS betweenframes. A Request To Send (RTS) frame may be followed after SIFS with aClear To Send (CTS) frame, then the data may be transmitted a SIFS afterthe CTS, after which an ACK may follow the data after SIFS. As noted,such frame sequences are all interspersed with SIFS. The SIFS durationmay be used for (a) the detection of energy on the channel, and todetermine whether energy has gone away (i.e., the channel clears), (b)time to decode the previous message and determine whether an ACK framewill indicate the transmission was received correctly, and (c) time forthe STA transceivers to switch from receive to transmit, and vice versa.

FIG. 5 depicts example physical layer (PHY) transmission segment 500,which illustrates the use of SIFS before an ACK, with higher prioritythan a DIFS access. An existing transmission 510 utilizes the channel.When transmission 510 terminates, in this example, ACK 520 follows theend of transmission 510 after a SIFS. Note that ACK 520 begins before aDIFS expires, thus any other STAs attempting to earn a transmissionwould not succeed. In this example, after the ACK 520 completes, nohigher priority accesses occur, and so new transmission 530 begins afterDIFS and the associated backoff period, if any.

The RTS/CTS frame sequence (in addition to providing flow controlfeatures) may be used to improve protection for the data frametransmission. The RTS and CTS contain duration information for thesubsequent data frame and ACK and any intervening SIFS. STAs hearingeither the RTS or the CTS mark out the occupied duration on theirNetwork Allocation Vector (NAV) and treat the medium as busy for theduration. Typically, frames longer than a specified length are protectedwith RTS/CTS, while shorter frames are transmitted unprotected.

The PCF may be used to allow an AP to provide centralized control of thechannel. An AP may gain control of the medium after sensing the mediumto be idle for a PIFS duration. The PIFS is shorter than the DIFS andthus has higher priority than DIFS. Once the AP has gained access to thechannel it can provide contention-free access opportunities to otherSTAs and thus improve MAC efficiency compared to DCF. Note that SIFS hashigher priority than PIFS, so the PCF must wait until any SIFS sequencescomplete before taking control of the channel.

Once the AP gains access to the medium using the PIFS it can establish aContention-Free Period (CFP) during which the AP can provide polledaccess to associated STAs. The contention-free poll (CF-Poll), or simplypoll, is transmitted by the AP and is followed by a transmission fromthe polled STA to the AP. Once again, the STA must wait for a SIFSduration following the CF-Poll, although the polled STA need not waitfor DIFS, or any backoff. 802.11(e) introduced various enhancements,including enhancements to polling, an example of which is detailedfurther below with respect to FIG. 9.

The Beacon transmitted by the AP establishes the duration of the CFP.This is similar to using RTS or CTS to prevent contention access.However, hidden terminal problems can still occur from terminals thatare unable to hear the Beacon, but whose transmissions may interferewith transmissions scheduled by the AP. Further protection is possiblethrough the use of a CTS-to-self by each terminal that begins atransmission in the CFP.

ACKs and CF-Polls are permitted to be included in one frame, and may beincluded with data frames to improve MAC efficiency. Note that theSIFS<PIFS<DIFS relationship provides a deterministic priority mechanismfor channel access. The contention access between STAs in the DCF isprobabilistic based on the back-off mechanism.

Early 802.11 standards also provided for segmenting large packets intosmaller fragments. One benefit of such segmenting is that an error in asegment requires less retransmission than an error in a larger packet.One drawback of segmenting in these standards is, for acknowledgedtransmission, the requirement of transmitting an ACK for each segment,with the additional SIFS that correspond to the additional ACKtransmissions and fragment transmissions. This is illustrated in FIG. 6.The example physical layer (PHY) transmission segment 600 illustratesthe transmission of N segments and their respective acknowledgement.Existing transmission 610 is transmitted. At the end of transmission610, a first STA waits DIFS 620 and backoff 630 to earn access to thechannel. The first STA transmits N fragments 640A-640N to a second STA,after which N respective delays of SIFS 650A-650N must transpire. Thesecond STA transmits N ACK frames 660A-660N. Between each fragment, thefirst STA must wait SIFS, so there are N−1 SIFS 670A-670N-1 as well.Thus, in contrast to sending one packet, one ACK, and one SIFS, asegmented packet requires the same time of packet transmission, with NACKs and 2N-1 SIFS.

The 802.11(e) standard adds enhancements to improve on the prior MACfrom 802.11(a), (b), and (g). 802.11(g) and (a) are both OFDM systems,which are very similar, but operate in different bands. Various featuresof lower speed MAC protocols, such as 802.11(b), were carried forward tosystems with much higher bit rates, introducing ineffiencies, detailedfurther below.

In 802.11(e), the DCF is enhanced and referred to as the EnhancedDistributed Channel Access (EDCA). The primary Quality of Service (QoS)enhancements of the EDCA are the introduction of an ArbitrationInterframe Spacing (AIFS). AIFS[i] is associated with a Traffic Class(TC) identified with index i. The AP may use AIFS[i] values differentfrom the AIFS[i] values that are allowed to be used by the other STAs.Only the AP may use an AIFS[i] value that is equal to the PIFS.Otherwise AIFS[i] is greater than or equal to DIFS. By default, the AIFSfor “voice” and “video” traffic classes is chosen to be equal to DIFS. Alarger AIFS implying lower priority is chosen for traffic classes “besteffort” and “background”.

The size of contention window is also made a function of the TC. Thehighest priority class is permitted to set the CW=1, i.e., no backoff.For other TCs, the different contention window sizes provide aprobabilistic relative priority, but cannot be used to achieve delayguarantees.

802.11(e) introduced the Transmission Opportunity (TXOP). To improve MACefficiency, when a STA acquires the medium through EDCA or through apolled access in HCCA, the STA may be permitted to transmit more than asingle frame. The one or more frames are referred to as the TXOP. Themaximum length of a TXOP on the medium depends on the traffic class andis established by the AP. Also, in the case of a polled TXOP, the APindicates the permitted duration of the TXOP. During the TXOP, the STAcan transmit a series of frames, interspersed with SIFS and ACKs fromthe destination. In addition to removing the need to wait DIFS plusbackoff for each frame, the STA having earned a TXOP has certainty thatit can retain the channel for subsequent transmissions.

During the TXOP, ACKs from the destination may be per frame (as inearlier 802.11 MACS), or may use an immediate or delayed block ACK asdiscussed below. Also, a no ACK policy is permitted for certain trafficflows, e.g., broadcast or multicast.

FIG. 7 depicts example physical layer (PHY) transmission segment 700,illustrating a TXOP with per-frame acknowledgment. An existingtransmission 710 is transmitted. Following the transmission 710, andafter waiting DIFS 720 and backoff 730, if any, a STA earns TXOP 790.TXOP 790 comprises N frames 740A-740N, each frame followed by Nrespective SIFS 750A-750N. The receiving STA responds with N respectiveACKS 760A-760N. The ACKs 760 are followed by N−1 SIFS 770A-770N−1. Notethat each frame 740 comprises a preamble 770 as well as header andpacket 780. Example embodiments, detailed below, allow for greatlyreducing the amount of transmission time reserved for preambles.

FIG. 8 illustrates a TXOP 810 with block acknowledgment. The TXOP 810may be earned through contention or polling. TXOP 810 comprises N frames820A-820N, each frame followed by N respective SIFS 830A-830N. Followingthe transmission of frames 820 and SIFS 830, a block ACK request 840 istransmitted. The receiving STA responds to the block ACK request at atime in the future. The Block ACK may be immediate following thecompletion of the transmission of a block of frames, or may be delayedto permit receiver processing in software.

Example embodiments, detailed below, allow for greatly reducing theamount of transmission time between frames (SIFS in this example). Insome embodiments, there is no need to delay between consecutivetransmissions (i.e. frames).

Note that, in 802.11(a) and other standards, for certain transmissionformats, a signal extension is defined which adds additional delay tothe end of each frame. While not technically included in the definitionof SIFS, various embodiments, detailed below, also allow for the removalof the signal extensions.

The Block ACK feature provides improved efficiency. In one example, upto 64 MAC Service Data Units (SDUs) (each possibly fragmented to 16fragments) corresponding to 1024 frames may be transmitted by a STA,while the destination STA is permitted to provide a single response atthe end of the block of frames indicating the ACK status of each of the1024 frames. Typically, at high rates, the MAC SDU will not befragmented, and for low latency, fewer than 64 MAC SDUs may betransmitted before requiring a Block ACK from the destination. In such acase, to transmit M frames, the total time is reduced from M frames+MSIFS+M ACKs+M−1 SIFS, to M frames+M SIFS+Block ACK. Embodiments detailedbelow improve on the block ACK efficiency even further.

The Direct Link Protocol (DLP), introduced by 802.11(e), allows a STA toforward frames directly to another destination STA within a BasicService Set (BSS) (controlled by the same AP). The AP may make a polledTXOP available for this direct transfer of frames between STAs. Prior tothe introduction of this feature, during polled access, the destinationof frames from the polled STA was always the AP, which would in turnforward the frames to the destination STA. By eliminating the two-hopframe forwarding, medium efficiency is improved. Embodiments detailedfurther below add substantial efficiency to DLP transfers.

802.11(e) also introduces an enhanced PCF, called the HybridCoordination Function (HCF). In HCF Controlled Channel Access (HCCA),the AP is allowed to access the channel at any time either to establisha Controlled Access Phase (CAP), which is like the CFP and is used toprovide transmission opportunities at any time during the contentionphase, not only immediately following the Beacon. The AP accesses themedium by waiting for a PIFS with no back-off

FIG. 9 depicts example physical layer (PHY) transmission segment 800,illustrating a polled TXOP using HCCA. In this example, the AP contendsfor the poll. An existing transmission 910 is transmitted. Following thetransmission 910, the AP waits PIFS, and then transmits poll 920,addressed to a STA. Note that other STAs contending for the channelwould have to wait at least DIFS, which does not occur due to thetransmitted poll 920, as shown. The polled STA transmits polled TXOP 940following the poll 920 and SIFS 930. The AP may continue to poll,waiting PIFS between each polled TXOP 940 and poll 920. In an alternatescenario, the AP may establish a CAP by waiting PIFS from a transmission910. The AP may transmit one or more polls during the CAP.

MAC Improvements

As described above, various inefficient features of prior MACs werebrought forward to later versions. For example, very long preambles,designed for 11 Mbps vs. 64 Mbps, introduce inefficiency. As the MACProtocol Data Unit (MPDU) keeps shrinking as rates increase, keeping thevarious interframe spacings and/or preambles constant means acorresponding decrease in channel utilization. For example, a high datarate MIMO MPDU transmission may be just a few microseconds in length,compared to 802.11(g), which has a 72 μs preamble. Eliminating orreducing delays, such as SIFS, signal extensions, and/or preambles willincrease throughput and utilization of the channel.

FIG. 10 is an example embodiment of a TXOP 1010 including multipleconsecutive transmissions without any gaps. TXOP 1010 comprises N frames1020A-1020N which are transmitted sequentially without any gaps (comparethis with the SIFS required in TXOP 810, depicted in FIG. 8). The numberof frames in the TXOP is limited only by the buffer and the decodingcapability of the receiver. When a STA is transmitting consecutiveframes with a Block ACK in a TXOP 1010, it is unnecessary to intersperseSIFS durations since no other STA needs to gain access to the medium inbetween consecutive frames. An optional block ACK request 1030 isappended to the N frames. Certain classes of traffic may not requireacknowledgement. A block ACK request may be responded to immediatelyfollowing the TXOP, or may be transmitted at a later time. The frames1020 do not require signal extensions. TXOP 1010 may be deployed in anyof the embodiments detailed herein where a TXOP is called for.

As shown in FIG. 10, the transmission of SIFS between consecutive framesin a TXOP, when all frames are transmitted by the same STA, may beeliminated. In 802.11(e), such gaps were retained to limit thecomplexity requirement at the receiver. In the 802.11(e) standard, the10 μs SIFS period and the 6 μs OFDM signal extension provide thereceiver with a total of 16 μs for processing the received frame(including demodulation and decoding). However, at large PHY rates, this16 μs results in significant inefficiency. In some embodiments, with theintroduction of MIMO processing, even the 16 μs may be insufficient tocomplete processing. Instead, in this example embodiment, the SIFS andOFDM signal extension between consecutive transmissions from one STA tothe AP or to another STA (using the Direct Link Protocol) areeliminated. Thus, a receiver requiring an additional period after thecompletion of the transmission, for MIMO receiver processing and channeldecoding (e.g. turbo/convolutional/LDPC decoding) may perform thosefunctions while the medium is utilized for additional transmission. Anacknowledgment may be transmitted at a later time, as described above(using block ACK, for example).

Due to different propagation delays between STAs, transmissions betweendifferent pairs of STAs may be separated by guard periods to avoidcollisions at a receiver between consecutive transmissions on the mediumfrom different STAs (not shown in FIG. 10, but detailed further below).In an example embodiment, a guard period of one OFDM symbol (4 μs) issufficient for all operating environments for 802.11. Transmissions fromthe same STA to different destination STAs do not need to be separatedby guard periods (as shown in FIG. 10). Detailed further below, theseguard periods may be referred to as Guardband Interframe Spacings(GIFS).

Instead of using SIFS and/or signal extension, the required receiverprocessing time (for MIMO processing and decoding, for example) may beprovided through the use of a window-based ARQ scheme (e.g. go back N orselective repeat), techniques known to those of skill in the art. Thestop-and-wait MAC layer ACK of legacy 802.11 has been enhanced in802.11(e) to a window-like mechanism with up to 1024 frames and BlockACK, in this example. It may be preferable to introduce a standardwindow-based ARQ mechanism rather than the ad-hoc Block ACK schemedesigned in 802.11(e).

The maximum permitted window may be determined by receiver processingcomplexity and buffering. The transmitter may be permitted to transmitenough data to fill the receiver window at the peak PHY rate achievablebetween the transmitter-receiver pair. For example, since the receiverprocessing may not be able to keep up with the PHY rate, the receivermay need to store soft demodulator outputs until they can be decoded.Therefore, the buffering requirements for physical layer processing atthe peak PHY rate may be used to determine the maximum permitted window.

In an example embodiment, the receiver may advertise the maximumpermitted PHY block size that it can process at a given PHY rate withoutoverflowing its physical layer buffers. Alternately, the receiver mayadvertise the maximum permitted PHY block size that it can process atthe maximum PHY rate without overflowing its physical layer buffers. Atlower PHY rates, longer block sizes may be processed without bufferoverflow. A known formula may be used by transmitters to compute themaximum permitted PHY block size for a given PHY rate, from theadvertised maximum permitted PHY block size at the maximum PHY rate.

If the advertised maximum PHY block size is a static parameter, then theamount of time before the physical layer buffers may be processed andthe receiver is ready for the next PHY burst is another receiverparameter that may be known at the transmitter and also at thescheduler. Alternately, the advertised maximum PHY block size may bevaried dynamically according to the occupancy of the physical layerbuffers.

The receiver processing delay may be used to determine the round-tripdelay for the ARQ, which in turn may be used to determine the delaysseen by the applications. Therefore, to enable low-latency services, thepermitted PHY block size may be limited.

FIG. 11 depicts an example embodiment of a TXOP 1110 illustratingreducing the amount of preamble transmission required. TXOP 1110comprises preamble 1120 followed by N consecutive transmissions1130A-1130N. An optional block ACK request 1140 may be appended. In thisexample, a transmission 1130 comprises a header and a packet. ContrastTXOP 1110 with TXOP 790 of FIG. 7, in which each frame 740 comprises apreamble, in addition to the header and packet. By sending a singlepreamble, the required preamble transmission is one preamble instead ofN preambles, for the same amount of transmitted data.

Thus, the preamble 1120 may be eliminated from successive transmissions.The initial preamble 1120 may be used by the receiver to acquire thesignal, as well as for fine frequency acquisition for OFDM. For MIMOtransmissions, the initial preamble 1120 may be extended compared to thecurrent OFDM preamble to enable the receiver to estimate the spatialchannels. However, subsequent frames within the same TXOP may notrequire additional preambles. Pilot tones within the OFDM symbols aregenerally sufficient for signal tracking. In an alternate embodiment,additional (preamble-like) symbols may be interspersed periodicallyduring the TXOP 1110. However, the overall preamble overhead may besignificantly reduced. The preamble may be sent only as necessary, andmay be sent differently based on the amount of time elapsed since apreviously transmitted preamble.

Note that the TXOP 1110 may incorporate features of legacy systems aswell. For example, the block ACK is optional. More frequent ACKs may besupported. Even so, a lesser gap, such as GIFS, may be substituted forthe longer SIFS (plus signal extension, if used). The consecutivetransmissions 1130 may also include segments of a larger packet, asdescribed above. Note further that the header for consecutivetransmissions 1130 to the same receiving STA may be compressed. Anexample of compressing headers is detailed further below.

FIG. 12 depicts an example embodiment of a method 1200 for incorporatingvarious aspects just described, including consolidating preambles,removing gaps such as SIFS, and inserting GIFs as appropriate. Theprocess begins in block 1210, where a STA earns a TXOP using any of thetechniques detailed herein. In block 1220, a preamble is transmitted asnecessary. Again, the preamble may be longer or shorter than a legacypreamble, and may vary depending on various parameters such as timeelapsed since the last transmitted preamble as necessary to enable thereceiving STA to estimate the MIMO spatial channel. In block 1230, theSTA transmits one or more packets (or, more generally, consecutivetransmissions of any kind), to a destination. Note that additionalpreambles need not be transmitted. In an alternate embodiment, one ormore additional preambles may optionally be transmitted, or apreamble-like symbol may be interspersed as desired. In block 1240, theSTA may optionally transmit to an additional receiving STA. In thiscase, a GIFS is inserted as necessary, and one or more consecutivetransmissions may be transmitted to the additional receiving STA. Thenthe process may stop. In various embodiments, the STA may continue totransmit to more than two STAs, inserting GIFS and/or preambles asrequired for the desired level of performance.

Hence, as described above, MAC efficiency may be further improved byconsolidating transmissions from a STA to multiple destination STAs intoconsecutive transmissions, thus eliminating many or all of the guardperiods and reducing preamble overhead. A single preamble (or pilottransmission) may be used for multiple consecutive transmissions fromthe same STA to different destination STAs.

Additional efficiency may be gained through poll consolidation. In oneexample embodiment, several polls may be consolidated into a controlchannel, examples of which are detailed below. In one example, the APmay transmit to multiple destination STAs a signal including pollmessages to assign TXOPs. By contrast, in 802.11(e), each TXOP ispreceded by a CF-Poll from the AP followed by a SIFS. Improvedefficiency results when several such CF-Poll messages are consolidatedinto a single control channel message (referred to as a SCHED message inan example embodiment, detailed below) used to assign several TXOPs. Ina general embodiment, any period of time may be allocated forconsolidated polls and their respective TXOPs. An example embodiment isdetailed below with respect to FIG. 15, and further examples are alsoincluded herein.

A control channel (i.e. SCHED) message may be encoded with a tiered ratestructure to further improve efficiency. Accordingly, a poll message toany STA may be encoded according to the channel quality between the APand the STA. The order of transmission of poll messages need not be theorder of the assigned TXOPs, but may be ordered according to codingrobustness.

FIG. 13 depicts example physical layer (PHY) transmission segment 1300,illustrating consolidated polls and their respective TXOPs. Consolidatedpolls 1310 are transmitted. The polls may be transmitted using a controlchannel structure, examples of which are detailed herein, or may betransmitted using myriad alternate techniques, which will be readilyapparent to one of skill in the art. In this example, to eliminate theneed for interframe spacing between the polls and any forward linkTXOPs, forward link TXOPs 1320 are transmitted directly after theconsolidated polls 1310. Subsequent to the forward link TXOPs 1320,various reverse link TXOPs 1330A-1330N are transmitted, with GIFS 1340inserted as appropriate. Note that GIFS need not be included whensequential transmissions from one STA are made (similar to the lack ofGIFS requirement for forward link transmissions emanating from the AP tovarious STAs). In this example, reverse link TXOPs include STA to STA(i.e. peer to peer) TXOPs (using DLP, for example). Note that the orderof transmission shown is for illustration only. Forward and reverse linkTXOPs (including peer to peer transmission) may be interchanged, orinterspersed. Some configurations may not results in the elimination ofas many gaps as other configurations. Those of skill in the art willreadily adapt myriad alternate embodiments in light of the teachingherein.

FIG. 14 depicts an example embodiment of a method 1400 for consolidatingpolls. The process begins in block 1410, where channel resources areallocated into one or more TXOPs. Any scheduling function may bedeployed to make the TXOP allocation determination. In block 1420, pollsfor assigning TXOPs according to the allocation are consolidated. Inblock 1430, the consolidated polls are transmitted to one or more STAson one or more control channels (i.e. the CTRLJ segments of the SCHEDmessage, in an example embodiment detailed below). In an alternateembodiment, any messaging technique may be deployed to transmit theconsolidated polls. In block 1440, STAs transmit TXOPs according to thepolled allocations in the consolidated polls. Then the process may stop.This method may be deployed in conjunction with consolidated pollintervals of any length, which may comprise all or part of the systemBeacon interval. Consolidated polling may be used intermittently withcontention based access, or legacy polling, as described above. In anexample embodiment, method 1400 may be repeated periodically, or inaccordance with other parameters, such as system loading or datatransmission demand.

An example embodiment of a MAC protocol illustrating various aspects isdetailed with respect to FIGS. 15 and 16. This MAC protocol is detailedfurther in co-pending U.S. patent application Ser. Nos. 10/964,237,10/964,332, and 10/964,320 entitled “WIRELESS LAN PROTOCOL STACK,” filedconcurrently herewith, assigned to the assignee of the presentinvention.

An example TDD MAC frame interval 1500 is illustrated in FIG. 15. Theuse of the term TDD MAC frame interval in this context refers to theperiod of time in which the various transmission segments detailed beloware defined. The TDD MAC frame interval 1500 is distinguished from thegeneric use of the term frame to describe a transmission in an 802.11system. In 802.11 terms, TDD MAC frame interval 1500 may be analogous tothe Beacon interval or a fraction of the Beacon interval. The parametersdetailed with respect to FIGS. 15 and 16 are illustrative only. One ofordinary skill in the art will readily adapt this example to myriadalternate embodiments, using some or all of the components described,and with various parameter values. MAC function 1500 is allocated amongthe following transport channel segments: broadcast, control, forwardand reverse traffic (referred to as the downlink phase and uplink phase,respectively), and random access.

In the example embodiment, a TDD MAC frame interval 1500 is TimeDivision Duplexed (TDD) over a 2 ms time interval, divided into fivetransport channel segments 1510-1550 as shown. Alternate orders anddiffering frame sizes may be deployed in alternate embodiments.Durations of allocations on the TDD MAC frame interval 1500 may bequantized to some small common time interval.

The example five transport channels within TDD MAC frame interval 1500include: (a) the Broadcast Channel (BCH) 1510, which carries theBroadcast Control Channel (BCCH); (b) the Control Channel (CCH) 1520,which carries the Frame Control Channel (FCCH) and the Random AccessFeedback Channel (RFCH) on the forward link; (c) the Traffic Channel(TCH), which carries user data and control information, and issubdivided into (i) the Forward Traffic Channel (F-TCH) 1530 on theforward link and (ii) the Reverse Traffic Channel (R-TCH) 1540 on thereverse link; and (d) the Random Access Channel (RCH) 1550, whichcarries the Access Request Channel (ARCH) (for UT access requests). Apilot beacon is transmitted as well in segment 1510.

The downlink phase of frame 1500 comprises segments 1510-1530. Theuplink phase comprises segments 1540-1550. Segment 1560 indicates thebeginning of a subsequent TDD MAC frame interval. An alternateembodiment encompassing peer-to-peer transmission is illustrated furtherbelow.

The Broadcast Channel (BCH) and beacon 1510 is transmitted by the AP.The first portion of the BCH 510 contains common physical layeroverhead, such as pilot signals, including timing and frequencyacquisition pilot. In an example embodiment, the beacon consists of 2short OFDM symbols used for frequency and timing acquisition by the UTsfollowed by 8 short OFDM symbols of common MIMO pilot used by the UTs toestimate the channel.

The second portion of the BCH 1510 is the data portion. The BCH dataportion defines the allocation of the TDD MAC frame interval withrespect to the transport channel segments: CCH 1520, F-TCH 1530, R-TCH1540 and RCH 1550, and also defines the composition of the CCH withrespect to subchannels. In this example, the BCH 1510 defines thecoverage of the wireless LAN 120, and so is transmitted in the mostrobust data transmission mode available. The length of the entire BCH isfixed. In an example embodiment, the BCH defines the coverage of aMIMO-WLAN, and is transmitted in Space Time Transmit Diversity (STTD)mode using rate 1/4 coded Binary Phase Shift Keying (BPSK). In thisexample, the length of the BCH is fixed at 10 short OFDM symbols.Various other signaling techniques may be deployed in alternateembodiments.

The Control Channel (CCH) 1520, transmitted by the AP, defines thecomposition of the remainder of the TDD MAC frame interval, andillustrates the use of consolidated polls. The CCH 1520 is transmittedusing highly robust transmission modes in multiple subchannels, eachsubchannel with a different data rate. The first subchannel is the mostrobust and is expected to be decodable by all the UTs. In an exampleembodiment, rate 1/4 coded BPSK is used for the first CCH sub-channel.Several other subchannels with decreasing robustness (and increasingefficiency) are also available. In an example embodiment, up to threeadditional sub-channels are used. Each UT attempts to decode allsubchannels in order until a decoding fails. The CCH transport channelsegment in each frame is of variable length, the length depending on thenumber of CCH messages in each subchannel. Acknowledgments for reverselink random access bursts are carried on the most robust (first)subchannel of the CCH.

The CCH contains assignments of physical layer bursts on the forward andreverse links, (analogous to consolidated polls for TXOPs). Assignmentsmay be for transfer of data on the forward or reverse link. In general,a physical layer burst assignment comprises: (a) a MAC ID; (b) a valueindicating the start time of the allocation within the frame (in theF-TCH or the R-TCH); (c) the length of the allocation; (d) the length ofthe dedicated physical layer overhead; (e) the transmission mode; and(f) the coding and modulation scheme to be used for the physical layerburst.

Other example types of assignments on the CCH include: an assignment onthe reverse link for the transmission of a dedicated pilot from a UT, oran assignment on the reverse link for the transmission of buffer andlink status information from a UT. The CCH may also define portions ofthe frame that are to be left unused. These unused portions of the framemay be used by UTs to make noise floor (and interference) estimates aswell as to measure neighbor system beacons.

The Random Access Channel (RCH) 1550 is a reverse link channel on whicha UT may transmit a random access burst. The variable length of the RCHis specified for each frame in the BCH.

The Forward Traffic Channel (F-TCH) 1530 comprises one or more physicallayer bursts transmitted from the AP 104. Each burst is directed to aparticular MAC ID as indicated in the CCH assignment. Each burstcomprises dedicated physical layer overhead, such as a pilot signal (ifany) and a MAC PDU transmitted according to the transmission mode andcoding and modulation scheme indicated in the CCH assignment. The F-TCHis of variable length. In an example embodiment, the dedicated physicallayer overhead may include a dedicated MIMO pilot. An example MAC PDU isdetailed with respect to FIG. 16.

The Reverse Traffic Channel (R-TCH) 1540 comprises physical layer bursttransmissions from one or more UTs 106. Each burst is transmitted by aparticular UT as indicated in the CCH assignment. Each burst maycomprise a dedicated pilot preamble (if any) and a MAC PDU transmittedaccording to the transmission mode and coding and modulation schemeindicated in the CCH assignment. The R-TCH is of variable length.

In the example embodiment, the F-TCH 530, the R-TCH 540, or both, mayuse spatial multiplexing or code division multiple access techniques toallow simultaneous transmission of MAC PDUs associated with differentUTs. A field containing the MAC ID with which the MAC PDU is associated(i.e. the sender on the uplink, or the intended recipient on thedownlink) may be included in the MAC PDU header. This may be used toresolve any addressing ambiguities that may arise when spatialmultiplexing or CDMA are used. In alternate embodiments, whenmultiplexing is based strictly on time division techniques, the MAC IDis not required in the MAC PDU header, since the addressing informationis included in the CCH message allocating a given time period in the TDDMAC frame interval to a specific MAC ID. Any combination of spatialmultiplexing, code division multiplexing, time division multiplexing,and any other technique known in the art may be deployed.

FIG. 16 depicts the formation of an example MAC PDU 1660 from a packet1610, which may be an IP datagram or an Ethernet segment, in thisexample. Example sizes and types of fields are described in thisillustration. Those of skill in the art will recognize that variousother sizes, types, and configurations are contemplated within the scopeof the present invention.

As shown, the data packet 1610 is segmented at an adaptation layer. Eachadaptation sublayer PDU 1630 carries one of these segments 1620. In thisexample, data packet 1610 is segmented into N segments 1620A-N. Anadaptation sublayer PDU 1630 comprises a payload 1634 containing therespective segment 1620. A type field 1632 (one byte in this example) isattached to the adaptation sublayer PDU 1630.

A Logical Link (LL) header 1642 (4 bytes in this example) is attached tothe payload 1644, which comprises the adaptation layer PDU 1630. Exampleinformation for LL header 1642 includes a stream identifier, controlinformation, and sequence numbers. A CRC 1646 is computed over theheader 1642 and the payload 1644, and appended to form a logical linksublayer PDU (LL PDU) 1640. Logical Link Control (LLC) and Radio LinkControl (RLC) PDUs may be formed in similar fashion. LL PDUs 1640, aswell as LLC PDUs and RLC PDUs, are placed in queues (for example, a highQoS queue, a best effort queue, or control message queue) for service bya MUX function.

A MUX header 1652 is attached to each LL PDU 1640. An example MUX header1652 may comprise a length and a type (the header 1652 is two bytes inthis example). A similar header may be formed for each control PDU (i.e.LLC and RLC PDUs). The LL PDU 1640 (or LLC or RLC PDU) forms the payload1654. The header 1652 and payload 1654 form the MUX sublayer PDU (MPDU)1650 (MUX sublayer PDUs are also referred to herein as MUX PDUs).

Communication resources on the shared medium are allocated by the MACprotocol in a series of TDD MAC frame intervals, in this example. Inalternate embodiments, examples of which are detailed further below,these type of TDD MAC frame intervals may be interspersed with variousother MAC functions, including contention based or polled, and includinginterfacing with legacy systems using other types of access protocols.As described above, a scheduler may determine the size of physical layerbursts allocated for one or more MAC IDs in each TDD MAC frame interval(analogous to consolidated polled TXOPs). Note that not every MAC IDwith data to be transmitted will necessarily be allocated space in anyparticular TDD MAC frame interval. Any access control or schedulingscheme may be deployed within the scope of the present invention. Whenan allocation is made for a MAC ID, a respective MUX function for thatMAC ID will form a MAC PDU 1660, including one or more MUX PDUs 1650 forinclusion in the TDD MAC frame interval. One or more MUX PDUs 1660, forone or more allocated MAC IDs will be included in a TDD MAC frameinterval (i.e. TDD MAC frame interval 1500, detailed with respect toFIG. 15, above).

In an example embodiment, one aspect allows for a partial MPDU 1650 tobe transmitted, allowing for efficient packing in a MAC PDU 1660. Inthis example, the untransmitted bytes of any partial MPDUs 1650 leftover from a previous transmission may be included, identified by partialMPDU 1664. These bytes 1664 will be transmitted ahead of any new PDUs1666 (i.e. LL PDUs or control PDUs) in the current frame. Header 1662(two bytes in this example) includes a MUX pointer, which points to thestart of the first new MPDU (MPDU 1666A in this example) to betransmitted in the current frame. Header 1662 may also include a MACaddress.

The MAC PDU 1660 comprises the MUX pointer 1662, a possible partial MUXPDU 1664 at the start (left over from a previous allocation), followedby zero or more complete MUX PDUs 1666A-N, and a possible partial MUXPDU 1668 (from the current allocation) or other padding, to fill theallocated portion of the physical layer burst. The MAC PDU 1660 iscarried in the physical layer burst allocated to the MAC ID.

Thus, the example MAC PDU 1660 illustrates a transmission (or frame, in802.11 terminology), that may be transmitted from one STA to another,including portions of data from one or more flows directed to thatdestination STA. Efficient packing is achieved with the optional use ofpartial MUX PDUs. Each MAC PDU may be transmitted in a TXOP (using802.11 terminology), at a time indicated in the consolidated pollincluded in the CCH.

The example embodiment detailed in FIGS. 15-16 illustrates variousaspects, including consolidated polls, reduced preamble transmission,and elimination of gaps by sequentially transmitting physical layerbursts from each STA (including the AP). These aspects are applicable toany MAC protocol, including 802.11 systems. Detailed further below arealternate embodiments illustrating various other techniques forachieving MAC efficiency, as well as supporting peer-to-peertransmission, and integrating with and/or cooperating with existinglegacy protocols or systems.

As described above, various embodiments detailed herein may employchannel estimation and tight rate control. Enhanced MAC efficiency maybe gained through minimizing unnecessary transmission on the medium, butinadequate rate control feedback may, in some cases, reduce the overallthroughput. Thus, sufficient opportunities may be provided for channelestimation and feedback to maximize the transmitted rate on all MIMOmodes, in order to prevent the loss of throughput due to inadequatechannel estimation, which may offset any MAC efficiency gains.Therefore, as described above, and detailed further below, example MACembodiments may be designed to provide sufficient preamble transmissionopportunities, as well opportunities for receivers to provide ratecontrol feedback to the transmitter.

In one example, the AP periodically intersperses MIMO pilot in itstransmissions (at least every TP ms, where TP may be a fixed or variableparameter). Each STA may also begin its polled TXOP with a MIMO pilotthat may be used by other STAs and the AP to estimate the channel. Forthe case of a transmission to the AP or to another STA using the DirectLink Protocol (detailed further below), the MIMO pilot may be a steeredreference to help simplify receiver processing at the destination STA.

The AP may also provide opportunities to the destination STA to provideACK feedback. The destination STA may also use these feedbackopportunities to provide rate control feedback for available MIMO modesto the transmitting STA. Such rate control feedback is not defined inlegacy 802.11 systems, including 802.11(e). The introduction of MIMO mayincrease the total amount of rate control information (per MIMO mode).In some instances, to maximize the benefit of improvements in MACefficiency, these may be complemented by tight rate control feedback.

Another aspect introduced here, and detailed further below, is backloginformation and scheduling for STAs. Each STA may begin its TXOP with apreamble followed by a requested duration of the next TXOP. Thisinformation is destined for the AP. The AP collects information on thenext requested TXOP from several different STAs and determines theallocation of duration on the medium of TXOPs for a subsequent TDD MACframe interval. The AP may use different priority or QoS rules todetermine how to share the medium, or it may use very simple rules toproportionally share the medium according to the requests from the STAs.Any other scheduling technique may also be deployed. The allocations forthe TXOPs for the next TDD MAC frame interval are assigned in thesubsequent control channel message from the AP.

Designated Access Point

In embodiments detailed herein, a network may support operation with orwithout a true access point. When a true AP is present, it may beconnected, for example, to a wired fat pipe connection (i.e. cable,fiber, DSL or T1/T3, Ethernet) or a home entertainment server. In thiscase, the true AP may be the source and sink for the majority of dataflowing between devices in the network.

When no true AP exists, stations may still communicate with one anotherusing techniques like the Distributed Coordination Function (DCF) or802.11b/g/a or the Enhanced Distributed Channel Access of 802.11e, asdescribed above. As detailed further below, when additional resourcesare required, more efficient use of the medium may be accomplished witha centralized scheduling scheme. This network architecture might arise,for example, in a home where many different devices need to communicatewith one another (i.e. DVD-TV, CD-Amp-Speakers, etc.). In this case, thenetwork stations automatically designate one station to become the AP.Note that, as detailed below, an Adaptive Coordination Function (ACF)may be utilized with a designated access point, and may be deployed withcentralized scheduling, random access, ad-hoc communication, or anycombination thereof.

Certain, but not necessarily all, non-AP devices may have enhanced MACcapability and are suitable for operation as a designated AP. It shouldbe noted that not all devices need to be designed to be capable ofdesignated AP MAC capability. When QoS (e.g., guaranteed latency), highthroughput, and/or efficiency is critical, it may be necessary that oneof the devices in the network be capable of designated AP operation.

This means that designated AP capability will generally be associatedwith devices with higher capability, e.g., with one or more attributessuch as line power, large number of antennas and/or transmit/receivechains, or high throughput requirement. (Additional factors forselecting a designated AP are detailed further below.) Thus, a low-enddevice such as a low-end camera or phone need not be burdened withdesignated AP capability, while a high-end device such as high-end videosource or a high definition video display may be equipped withdesignated AP capability.

In a no-AP network, the designated AP assumes the role of the true APand may or may not have reduced functionality. In various embodiments, adesignated AP may perform the following: (a) establish the network BasicService Set (BSS) ID; (b) set network timing by transmitting a beaconand broadcast channel (BCH) network configuration information (the BCHmay define composition of the medium until the next BCH); (c) manageconnections by scheduling transmissions of stations on the network usinga Forward Control Channel (FCCH); (d) manage association; (e) provideadmission control for QoS flows; and/or (f) various other functions. Thedesignate AP may implement a sophisticated scheduler, or any type ofscheduling algorithm. A simple scheduler may be deployed, an example ofwhich is detailed further below.

A modified Physical Layer Convergence Protocol (PLCP) header is detailedbelow with respect to peer-peer communications, that is also applicablefor designated APs. In one embodiment, the PLCP header of alltransmissions is transmitted at the basic data rate that can be decodedby all stations (including the designated AP). The PLCP header oftransmissions from stations contains data backlog at the stationassociated with a given priority or flow. Alternately, it contains arequest for duration of a subsequent transmission opportunity for agiven priority or a flow.

The designated AP may determine backlog or transmission opportunitydurations requested by the stations by “snooping” in the PLCP Headers ofall station transmissions. The designated AP may determine the fractionof time to be allocated to EDCA-based (distributed access) and thefraction of time allocated to contention-free polled (centralized)access based on load, collisions, or other congestion measures. Thedesignated AP may run a rudimentary scheduler that allocates bandwidthin proportion to the requests and schedules them in the contention-freeperiod. Enhanced schedulers are permitted but not mandated. Thescheduled transmissions may be announced by the designated AP on the CCH(control channel).

A designated AP may not be required to echo one station's transmissionto another station (i.e. serve as a hop point), although thisfunctionality is allowed. A true AP may be capable of echoing.

When selecting a designated access point, a hierarchy may be created todetermine which device should serve as access point. Example factorsthat may be incorporated in selecting a designated access point includethe following: (a) user over-ride; (b) higher preference level; (c)security level; (d) capability: line power; (e) capability: number ofantennas; (f) capability: max transmit power; (g) to break a tie basedon other factors: Medium Access Control (MAC) address; (h) first devicepowered on; (i) any other factors.

In practice, it may be desirable for the designated AP to be centrallylocated and have the best aggregate Rx SNR CDF (i.e. be able to receiveall stations with a good SNR). In general, the more antennas a stationhas, the better the receive sensitivity. In addition, the designated APmay have a higher transmit power so that the designated AP may be heardby a large number of stations. These attributes can be assessed andexploited to allow the network to dynamically reconfigure as stationsare added and/or moved around.

Peer-to-peer connections may be supported in cases where the network isconfigured with a true AP or a designated AP. Peer-to-peer connections,in general, are detailed in the next section below. In one embodiment,two types of peer-to-peer connections may be supported: (a) managedpeer-to-peer, where the AP schedules transmissions for each stationinvolved; and (b) ad-hoc, where the AP is not involved in the managementor scheduling of station transmissions.

The designated AP may set the MAC frame interval and transmit a beaconat the start of the frame. The broadcast and control channels mayspecify allocated durations in the frame for the stations to transmit.For stations that have requested allocations for peer-to-peertransmissions (and these requests are known to the AP), the AP mayprovide scheduled allocations. The AP may announce these allocations inthe control channel, such as, for example, with every MAC frame.

Optionally, the AP may also include an A-TCH (ad hoc) segment in the MACframe (detailed further below). The presence of the A-TCH in the MACframe may be indicated in the BCH and FCCH. During the A-TCH, stationsmay conduct peer-to-peer communication using CSMA/CA procedures. TheCSMA/CA procedures of the IEEE Wireless LAN Standard 802.11 may bemodified to exclude the requirement for immediate ACK. A station maytransmit a MAC-PDU (Protocol Data Unit) consisting of multiple LLC-PDUswhen the station seizes the channel. The maximum duration that may beoccupied by a station in the A-TCH may be indicated in the BCH. Foracknowledged LLC, the window size and maximum acknowledgment delay maybe negotiated according to the required application delay. A modifiedMAC frame with an A-TCH segment, for use with both true APs anddesignated APs, is detailed further below with respect to FIG. 20.

In one embodiment, the unsteered MIMO pilot may enable all stations tolearn the channel between themselves and the transmitting station. Thismay be useful in some scenarios. Further, the designated AP may use theunsteered MIMO pilot to allow channel estimation and facilitatedemodulation of the PCCH from which allocations can be derived. Once thedesignated AP receives all requested allocations in a given MAC frame,it may schedule these for the subsequent MAC frame. Note that ratecontrol information does not have to be included in the FCCH.

In one embodiment, the scheduler may perform the following operations:First, the scheduler collects all the requested allocations for the nextMAC frame and computes the aggregate requested allocation (TotalRequested). Second, the scheduler computes the total resource availablefor allocation to the F-TCH and the R-TCH (Total Available). Third, ifTotal Requested exceeds Total Available, all requested allocations arescaled by the ratio defined by Total Available/Total Requested. Fourth,for any scaled allocations that are less than 12 OFDM symbols, theseallocations are increased to 12 OFDM symbols (in the example embodiment;alternate embodiments may be deployed with alternate parameters). Fifth,to accommodate the resulting allocations in the F-TCH+R-TCH, any excessOFDM symbols and/or guard times may be accommodated by reducing allallocations larger than 12 OFDM symbols, one symbol at a time inround-robin fashion starting from the largest.

An example illustrates the embodiment just described. Considerallocation requests as follows: 20, 40, 12, 48. Thus, TotalRequested=120. Assume that Total Available=90. Also assume that theguard time required is 0.2 OFDM symbols. Then, as detailed in the thirdoperation above, the scaled allocations are: 15, 30, 9, 36. As detailedin the fourth operation above, an allocation of 9 is increased to 12.According to the fifth operation, adding the revised allocations and theguard time, the total allocation is 93.8. This means that theallocations are to be reduced by 4 symbols. By starting with thelargest, and removing one symbol at a time, a final allocation of 14,29, 12, 34 is determined (i.e. a total of 89 symbols and 0.8 symbols forguard times).

In an example embodiment, when the designated AP is present, it mayestablish the Beacon for the BSS and set network timing. Devicesassociate with the designated AP. When two devices associated with adesignated AP require a QoS connection, e.g. a HDTV link with lowlatency and high throughput requirement, they provide the trafficspecification to the designated AP for admission control. The designatedAP may admit or deny the connection request.

If the medium utilization is sufficiently low, the entire duration ofthe medium between beacons may be set aside for EDCA operation usingCSMA/CA. If the EDCA operation is running smoothly, e.g., there are noexcessive collisions, back-offs and delays, the designated AP does notneed to provide a coordination function.

The designated AP may continue to monitor the medium utilization bylistening to the PLCP headers of station transmissions. Based onobserving the medium, as well as the backlog or transmission opportunityduration requests, the designated AP may determine when EDCA operationis not satisfying the required QoS of admitted flows. For example it mayobserve the trends in the reported backlogs or requested durations, andcompare them against the expected values based on the admitted flows.

When the Designated AP determines that the required QoS is not being metunder distributed access, it can transition operation on the medium tooperation with polling and scheduling. The latter provides moredeterministic latency and higher throughput efficiency. Examples of suchoperation are detailed further below.

Thus, adaptive transition from EDCA (distributed access scheme) toscheduled (centralized) operation as a function of the observation ofthe medium utilization, collisions, congestion, as well as, observationof the transmission opportunity requests from transmitting stations andcomparison of the requests against admitted QoS flows may be deployed.

As mentioned previously, in any embodiment detailed throughout thisspecification, where an access point is described, one of skill in theart will recognize that the embodiment may be adapted to operate with atrue access point or a designated access point. A designated accesspoint may also be deployed and/or selected as detailed herein, and mayoperate according to any protocol, including protocols not described inthis specification, or any combination of protocols.

Peer-to-Peer Transmission and Direct Link Protocol (DLP)

As described above, peer-to-peer (or simply referred to as “peer-peer”)transmission allows one STA to transmit data directly to another STA,without sending the data first to an AP. Various aspects detailed hereinmay be adopted for use with peer-to-peer transmission. In oneembodiment, the Direct Link Protocol (DLP) may be adapted as detailedfurther below. FIG. 17 depicts an example peer-to-peer communicationwithin a system 100. In this example, system 100, which may be similarto system 100 depicted in FIG. 1, is adapted to allow directtransmission from one UT to another (in this example, transmissionbetween UT 106A and UT 106B is illustrated). UTs 106 may perform anycommunication directly with AP 104 on WLAN 120, as detailed herein.

In various example embodiments, two types of peer-peer connections maybe supported: (a) Managed peer-peer, in which the AP schedulestransmissions for each STA involved, and (b) Ad-hoc, in which the AP isnot involved in the management or scheduling of STA transmissions. Anembodiment may include either or both types of connections. In anexample embodiment, a transmitted signal may comprise a portionincluding common information that is receivable by one or more stations,possibly including an access point, as well as information specificallyformatted for reception by a peer-peer receiving station. The commoninformation may be used for scheduling (as shown in FIG. 25, forexample) or for contention backoff by various neighbor stations (shownin FIG. 26, for example).

Various example embodiments, detailed below, illustrate closed loop ratecontrol for peer-peer connections. Such rate control may be deployed totake advantage of available high data rates.

For clarity of discussion, various features (i.e. acknowledgement) arenot necessarily detailed in example embodiments. Those of skill in theart will recognize that features disclosed herein may be combined toform any number of sets and subsets in various embodiments.

FIG. 18 depicts a prior art physical layer burst 1800. A preamble 1810may be transmitted, followed by a Physical Layer Convergence ProtocolHeader (PLCP) header 1820. Legacy 802.11 systems define a PLCP header toinclude rate type and modulation format for data transmitted as datasymbols 1830.

FIG. 19 depicts an example physical layer burst 1900, which may bedeployed for peer-peer transmission. As in FIG. 18, preamble 1810 andPLCP header 1820 may be included, followed by a peer-peer transmission,labeled P2P 1940. P2P 1940 may comprise a MIMO pilot 1910 for use by thereceiving UT. MIMO rate feedback 1920 may be included for use by thereceiving UT in future transmission back to the sending UT. Ratefeedback may be generated in response to a previous transmission fromthe receiving station to the transmitting station. Then data symbols1930 may be transmitted according to the selected rate and modulationformat for the peer-peer connection. Note that a physical layer burst,such as PHY burst 1900, may be used with AP managed peer-peerconnection, as well as with ad hoc peer-peer transmission. Example ratefeedback embodiments are described below. Alternate embodiments ofphysical layer transmission bursts including these aspects are alsoincluded below.

In an example embodiment, an AP sets the TDD MAC frame interval.Broadcast and control channels may be deployed to specify allocateddurations in the TDD MAC frame interval. For STAs that have requestedallocations for peer-peer transmissions (and known to the AP), the APmay provide scheduled allocations and announce these in the controlchannel every TDD MAC frame interval. An example system is describedabove with respect to FIG. 15.

FIG. 20 depicts an example embodiment of a TDD MAC frame interval 2000including an optional ad hoc segment, identified as A-TCH 2010. The likenumbered sections of TDD MAC frame interval 2000 may be included anoperate substantially as described above with respect to FIG. 15. Thepresence of the A-TCH 2010 in the TDD MAC frame interval 2000 may beindicated in the BCH 510 and/or CCH 520. During the A-TCH 2010, STAs mayconduct peer-to-peer communication using any contention procedure. Forexample, 802.11 techniques such as SIFS, DIFS, backoff, etc., asdetailed above may be deployed. QoS techniques, such as those introducedin 802.11(e) (i.e. AIFS) may optionally be deployed. Various othercontention based schemes may be deployed as well.

In an example embodiment, CSMA/CA procedures for contention, such asthose defined in 802.11, may be modified as follows. Immediate ACK isnot required. A STA may transmit a MAC Protocol Data Unit (MAC-PDU)consisting of multiple PDUs (i.e. LLC-PDUs) when it seizes the channel.A maximum duration occupied by a STA in the A-TCH may be indicated inthe BCH. When acknowledged transmission is desired, a window size andmaximum acknowledgment delay may be negotiated according to the requiredapplication delay.

In this example, the F-TCH 530 is the portion of the TDD MAC frameinterval for transmissions from the AP to STAs. Peer-to-peercommunications between STAs using contention techniques may be conductedin the A-TCH 2010. Scheduled peer-to-peer communications between STAsmay be conducted in the R-TCH 540. Any of these three segments may beset to null.

FIG. 21 depicts an example physical layer burst 2100, also referred toas a “PHY burst”. PHY burst 2100 may be deployed with scheduledpeer-peer connections, such as during R-TCH 540, or during ad hocconnections such as A-TCH 2010, as detailed above with respect to FIG.20. PHY burst 2100 comprises un-steered MIMO pilot 2110, Peer CommonControl Channel (PCCH) 2120, and one or more data symbols 2130. Theunsteered MIMO pilot 2110 may be received at one or more stations, andmay be used as a reference by a receiving station to estimate therespective channel between the transmitting station and the receivingstation. This example PCCH comprises the following fields: (a) adestination MAC-ID, (b) an allocation request for a desired transmissionduration for the next TDD MAC frame interval, (c) a transmission rateindicator to indicate the transmission format for the current datapacket, (d) a control channel (i.e. CCH) subchannel for receiving anyallocation from the AP, and (e) a CRC. The PCCH 2120, along withun-steered MIMO pilot 2110, is a common segment that may be received byvarious listening stations, including the access point. A request forallocation may be inserted in the PCCH to allow for a managed peer-peerconnection in a future TDD MAC frame interval. Such a PHY burst may beincluded in an ad-hoc connection, and may still request an allocationfor scheduled peer to peer in a future TDD MAC frame interval. In theexample embodiment, the unsteered MIMO pilot is eight OFDM symbols (inalternate embodiments, detailed below, fewer symbols may be sufficientfor channel estimation) and the PCCH is two OFDM symbols. Following thecommon segment, comprising unsteered MIMO pilot 2110 and PCCH 2120, oneor more data symbols 2130 are transmitted using spatial multiplexingand/or higher modulation formats as determined by each STA in thepeer-peer connection. This portion of the transmission is codedaccording to rate control information embedded in the data portion ofthe transmission. Thus, a portion of the PHY burst 2100 is receivable bymultiple surrounding stations, while the actual data transmission istailored for efficient transmission to one or more specific peer-peerconnected stations or the AP. Data at 2130 may be transmitted asallocated by an access point, or may be transmitted in accordance withan ad-hoc connection (i.e. CSMA/CA contention based procedures).

An example embodiment of a PHY burst comprises a preamble consisting of8 OFDM symbols of un-steered MIMO reference. A Peer Common ControlChannel (PCCH) MAC-PDU header is included in the subsequent 2 OFDMsymbols, using STTD mode, encoded with R=½ BPSK. The MAC-ID is 12 bits.An 8-bit allocation request is included for reception by the AP for adesired duration in the next TDD MAC frame interval (thus the maximumrequest is 256 short OFDM symbols). The TX Rate is 16 bits to indicatethe rate being used in the current packet. The FCCH subchannelpreference is two bits, corresponding to a preference between up to foursubchannels, on which the AP should make any applicable allocation. TheCRC is 10 bits. Any number of other fields and/or field sizes may beincluded in an alternate PHY burst embodiment.

In this example, the remainder of the MAC-PDU transmission uses spatialmultiplexing and higher modulations as determined by each STA in thepeer-peer connection. This portion of the transmission is codedaccording to the rate control information embedded in the data portionof the transmission.

FIG. 22 depicts example method 2200 for peer-peer data transmission. Theprocess begins in block 2210 where a station transmits an unsteered MIMOpilot. In block 2220, the station transmits commonly decodableinformation. For example, unsteered MIMO pilot 2110 and PCCH 2120 serveas an example of a mechanism for requesting allocation in a managedconnection, for which the AP, or other scheduling station, would need tobe able to decode the portion of the signal comprising the request.Those of skill in the art will recognize myriad alternate requestmechanisms for scheduling peer-peer connections on a shared channel. Inblock 2230, data is transmitted from one station to another inaccordance with negotiated transmission formats. In this example,steered data is transmitted using rates and parameters as determined inaccordance with measurements of unsteered MIMO pilot 2110. Those ofskill in the art will recognize various alternate means for transmittingdata tailored for a specific peer-peer channel.

FIG. 23 depicts example method 2300 for peer-peer communication. Thisexample method 2300 illustrates several aspects, subsets of which may bedeployed in any given embodiment. The process begins in decision block2310. In decision block 2310, if there is data for STA-STA transfer,proceed to decision block 2320. If not, proceed to block 2370 andperform any other type of communication, including other access types,if any. Proceed to decision block 2360 where the process may repeat byreturning to decision block 2310, or the process may stop.

In decision block 2320, if there is STA-STA data for transmission,determine whether the peer-peer connection is to be scheduled or ad hoc.If the transmission is to be scheduled proceed to block 2320 and requestan allocation to earn a TXOP. Note that an allocation request may bemade during a random access portion of a TDD MAC frame interval, asdescribed above, or may be included in an ad hoc transmission. Once anallocation is made, in block 2350 a STA-STA physical burst may betransmitted. In an example embodiment, method 2200 may serve as one typeof STA-STA PHY burst.

In decision block 2320, if scheduled peer-peer connection is notdesired, proceed to block 2340 to contend for access. For example, theA-TCH 2010 segment of TDD MAC frame interval 2000 may be used. When anaccess has been earned successfully through contention proceed to block2350 and transmit a STA-STA PHY burst, as described above.

From block 2350, proceed to decision block 2360 where the process mayrepeat, as described above, or may stop.

FIG. 24 depicts example method 2400 for providing rate feedback for usein peer-peer connection. This FIG illustrates various transmissions andother steps that may be performed by two stations, STA 1 and STA 2. STA1 transmits an unsteered pilot 2410 to STA 2. STA 2 measures the channel2420 while receiving unsteered pilot 2410. In an example embodiment STA2 determines a supportable rate for transmission on the channel asmeasured. This rate determination is transmitted as rate feedback 2430to STA 1. In various alternate embodiments, alternate parameters may bedelivered to allow for a rate feedback decision to be made at STA 1. At2440, STA 1 receives a scheduled allocation or contends for a transmitopportunity, for example during A-TCH. Once a transmit opportunity hasbeen earned, at 2450, STA 1 transmits to STA 2 data at a rate andmodulation format determined in response to rate feedback 2430.

The method illustrated in FIG. 24 may be generalized and applied tovarious embodiments, as will be readily apparent to those with skill inthe art. Some examples incorporating peer-peer rate feedback, as well asother aspects are detailed further below.

FIG. 25 depicts method 2500 illustrating managed peer-peer connectionbetween two stations, STA 1 and STA 2, and an access point (AP). At2505, STA 1 transmits an unsteered pilot as well as a request for anallocation. Data may also be transmitted according to an earlierallocation and previous rate feedback, as will be illustrated below.Further, any such data may be transmitted according to rate feedbackfrom a previous managed peer-peer connection or from ad hoccommunication originated by either STA 1 or STA 2. The unsteered pilotand request for transmission is received by both STA 2 and the accesspoint (and may be receivable by various other stations in the area).

The access point receives the request for transmission and, inaccordance with one of any number of scheduling algorithms, makes adetermination of when and whether to make an allocation for thepeer-peer communication. STA 2 measures the channel while the unsteeredpilot in 2505 is transmitted and may make a determination about thesupportable rate for peer-peer communication with STA 1. Optionally, STA2 may also receive rate feedback and/or data from STA 1 in accordancewith a previous transmission.

In this example, the access point has determined an allocation will bemade for the requested transmission. At 2515 an allocation istransmitted from the access point to STA 1. In this example, allocationson the R-TCH 540, are transmitted during the control channel, such asCCH 520, illustrated above. Similarly at 2520 an allocation on the R-TCHis made for STA 2. At 2525, STA 1 receives the allocation from theaccess point. At 2530 STA 2 receives the allocation from the accesspoint.

STA 2 transmits rate feedback at 2535, in accordance with allocation2520. Optionally, a request for scheduled transmission may be included,as described above, as well as any data to be transmitted in accordancewith a previous request. The rate feedback transmitted is selected inaccordance with the channel measurement 2510, as described above. ThePHY burst of 2535 may include an unsteered pilot as well. At 2540 STA 1measures the channel from STA 2, receives the rate feedback, and mayreceive optional data as well.

At 2545, in accordance with allocation 2515, STA 1 transmits data inaccordance with the rate feedback information received. In addition, arequest may be made for a future allocation as well as rate feedback inaccordance with the channel measurement at 2540. The data is transmittedaccording to the specific channel measurement for the peer-peercommunication. At 2550 STA 2 receives the data as well as any optionallytransmitted rate feedback. STA 2 may also measure the channel to providerate feedback for future transmissions.

Note that both transmissions 2535 and 2545 are receivable by the accesspoint, at least the unsteered portion, as described above. Thus for anyincluded request, the access point may make additional allocations forfuture transmissions as indicated by allocations 2555 and 2560 to STA 1and STA 2, respectively. At 2565 and 2570, STA 1 and STA 2 receive theirrespective allocations. The process may then iterate indefinitely withthe access point managing access on the shared medium and STA 1 and STA2 transmitting peer-peer communication directly to each other at ratesand modulation formats selected as supportable on the peer-peer channel.Note that, in an alternate embodiment, ad hoc peer-peer communicationmay also be performed along with the managed peer-peer communicationillustrated in FIG. 25.

FIG. 26 illustrates a contention based (or ad hoc) peer-peer connection.STA 1 and STA 2 will communicate with each other. Other STAs may also bein receiving range and may access the shared channel. At 2610 STA 1,having data to transmit to STA 2, monitors the shared channel andcontends for access. Once a transmit opportunity has been earned,peer-peer PHY burst 2615 is transmitted to STA 2 which may also bereceived by other STAs. At 2620, other STAs, monitoring the sharedchannel, may receive the transmission from STA 1 and know to avoidaccessing the channel. For example, a PCCH, described above, may beincluded in the transmission 2615. At 2630, STA 2 measures the channelin accordance with an unsteered pilot, and contends for return access onthe shared channel. STA 2 may also transmit data, as necessary. Notethat contention time may vary. For example, an ACK may be returnedfollowing SIFS in a legacy 802.11 system. Since SIFS is highestpriority, STA 2 may responds without losing the channel. Variousembodiments may allow for less delay, and may provide for return datawith high priority.

At 2635, STA 2 transmits rate feedback along with optional data to STA1. At 2640, STA 1 receives the rate feedback, contends once more foraccess to the shared medium, and transmits at 2645 to STA 2 inaccordance with the received rate feedback. At 2640, STA 1 may alsomeasure the channel to provide rate feedback to STA 2 for futuretransmission, and may receive any optional data transmitted by STA 2. At2650, STA 2 receives the data transmission 2645 in accordance with therate and modulation format determined by the measured channelconditions. STA 2 may also receive rate feedback for use in returning atransmission to STA 1. STA 2 may also measure the channel to providefuture rate feedback. The process may thus repeat by returning to 2635for STA 2 to return rate feedback as well as data.

Thus, two stations may perform ad hoc communication in both directionsby contending for access. The peer-peer connection itself is madeefficient by use of rate feedback and tailoring the transmission to thereceiving station. When a commonly receivable portion of the PHY burst,such as the PCCH, is deployed, then, as illustrated in 2620, other STAsmay access the information and may avoid interfering on the channel attimes known to be occupied, as indicated in the PCCH. As with FIG. 25,either managed or ad hoc peer-peer communication may initiate datatransfer prior to the steps illustrated in FIG. 26, and may be used tocontinue peer-peer communication subsequently. Thus, any combination ofscheduled and ad hoc peer-peer communication may be deployed.

FIG. 27 depicts example TDD MAC frame interval 2700, illustratingmanaged peer-peer communication between stations. In this example, boththe F-TCH and the A-TCH durations have been set to zero. Beacon/BCH 510and CCH 520 are transmitted as before. Beacon/BCH 560 indicates thestart of the next frame. CCH 520 indicates allocations for peer-peercommunications. In accordance with those allocations, STA 1 transmits toSTA 2 in allocated burst 2710. Note that, in the same TDD MAC frameinterval, STA 2 is allocated segment 2730 for responding to STA 1. Anyof the various components, detailed above, such as rate feedback,requests, steered and/or unsteered pilots, and steered and/or unsteereddata may be included in any given peer-peer PHY layer burst. STA 3transmits to STA 4 in allocation 2720. STA 4 transmits to STA 3 inallocation 2740, in similar fashion. Various other reverse linktransmissions, including non peer-peer connections, may be included inthe R-TCH. Additional example embodiments illustrating these and otheraspects are detailed further below.

Note that, in FIG. 27, guard intervals may be scheduled betweensegments, as necessary. A key issue regarding peer-peer communicationsis that generally the path delay between the two STAs is unknown. Onemethod of handling this is to make each STA keep its transmit timesfixed so that they arrive at the AP in synch with the AP's clock. Inthis case, the AP may provide for guard time on either side of eachpeer-to-peer allocation to compensate for unknown path delays betweenthe two communicating STAs. In many cases, a cyclic prefix will beadequate and no adjustments will need to be made at the STA receivers.The STAs must then determine their respective time offsets to know whento receive the other STA's transmission. The STA receivers may need tomaintain two receive clocks: one for the AP frame timing and another forthe peer-peer connection.

As illustrated in various embodiments above, acknowledgments and channelfeedback may be derived by a receiver during its allocation and fed backto a transmitter. Even if the overall traffic flow is one-way, thereceiver sends reference and requests to obtain allocations. The APscheduler ensures that adequate resources for feedback are provided.

Interoperability with Legacy Stations and Access Points

As detailed herein, various embodiments described provide improvementsover legacy systems. Nonetheless, given the wide deployment of legacysystems already in existence, it may be desirable for a system to retainbackward compatibility with either an existing legacy system and/orlegacy user terminals. As used herein, the term “new class” will be usedto differentiate from legacy systems. A new class system may incorporateone or more of the aspects or features detailed herein. An example newclass system is the MIMO OFDM system described below with respect toFIGS. 35-52. Furthermore, the aspects detailed below for interoperatinga new class system with a legacy system are also applicable to othersystems, yet to be developed, whether or not any particular improvementdetailed herein is included in such a system.

In one example embodiment, backward compatibility with alternate systemsmay be provided by using separate Frequency Assignments (FA) to allowthe operation of a new class system on a separate FA from legacy users.Thus, a new class system may search for an available FA on which tooperate. A Dynamic Frequency Selection (DFS) algorithm may beimplemented in the new class WLAN to accommodate this. It may bedesirable to deploy an AP to be multi-carrier.

Legacy STAs attempting to access a WLAN may employ two methods ofscanning: passive and active. With passive scanning, a STA develops alist of viable Basic Service Sets (BSSs) in its vicinity by scanning theoperating bands. With active scanning, a STA transmits a query tosolicit a response from other STAs in the BSS.

Legacy standards are silent as to how a STA decides which BSS to join,but, once a decision is made, association may be attempted. Ifunsuccessful, the STA will move through its BSS list until successful. Alegacy STA may not attempt to associate with a new class WLAN when thebeacon information transmitted would not be understood by that STA.However, a new class AP (as well as UTs) may ignore requests from legacySTAs as one method for maintaining a single WLAN class on a single FA.

An alternate technique is for new class AP or new class STAs to rejectany legacy STA's request using valid legacy (i.e., 802.11) messaging. Ifa legacy system supports such messaging, the legacy STA may be providedwith a redirection message.

An obvious tradeoff associated with operating on separate FAs is theadditional spectrum required to support both classes of STAs. Onebenefit is ease of management of the different WLANs preserving featuressuch as QoS and the like. As detailed throughout this specification,however, legacy CSMA MAC protocols (such as those detailed in the legacy802.11 standards), are generally inefficient for high data ratessupported for new class systems, such as the MIMO system embodimentdetailed herein. Thus, it is desirable to deploy backward compatiblemodes of operation allowing a new class MAC to co-exist with a legacyMAC on the same FA. Described below are several example embodiments inwhich legacy and new class systems may share the same FA.

FIG. 28 depicts method 2800 for supporting both legacy and new classstations on the same frequency assignment. In this example, for clarity,it is assumed that the BSS is operating in isolation (i.e., there is nocoordination between multiple overlapping BSSs). The process starts atblock 2810 where legacy signaling is used to establish a contention freeperiod.

Following are several illustrative examples, for use with legacy 802.11systems, in which the new class WLAN AP may use the hooks built into thelegacy 802.11 standard to reserve time for exclusive use by new classstations. Any number of additional signaling techniques, in addition tothese, may be used for establishing a contention free period, forvarious types of legacy systems.

One technique is to establish contention free periods (CFP) in PCF/HCFmode. The AP may establish a Beacon interval and announce a contentionfree period within the Beacon interval where it can serve both new classand legacy STAs in polled mode. This causes all legacy STAs to set theirNetwork Allocation Vectors (NAVs), which are counters used to keep trackof the CFP, to the duration of the announced CFP. As a result, legacySTAs that receive the beacon are prevented from using the channel duringthe CFP, unless polled by the AP.

Another technique is to establish a CFP, and setting NAV, via an RTS/CTSand duration/ID field. In this case, the new class AP may send out aspecial RTS which has a Reserved Address (RA) indicating to all newclass STAs that the AP is reserving the channel. Legacy STAs interpretthe RA field as being directed to a specific STA and do not respond. Thenew class STAs respond with a special CTS to clear out the BSS for thetime period specified in the duration/ID field in the CTS/RTS messagepair. At this point, the new class stations are free to use the channelfor the reserved duration without conflict.

In block 2820, legacy class STAs, having received the signal toestablish the contention free period, wait until polled or thecontention free period ends. Thus, the access point has successfullyallocated the shared medium for use with the new class MAC protocol. Inblock 2830, new STAs may access according to this protocol. Any set orsubset of the aspects detailed herein may be deployed in such a newclass MAC protocol. For example, scheduled forward and reverse linktransmissions as well as managed peer-peer transmissions, ad hoc orcontention based communication (including peer-peer), or any combinationof the above may be deployed. In block 2840, the new class access periodis terminated, using any of a variety of signal types, which may varyaccording to the legacy system deployed. In the example embodiment, acontention free period end signal is transmitted. In an alternateembodiment, legacy STAs may also be polled during a contention freeperiod. Such accesses may be subsequent to new class accesses, or may beinterspersed within them.

In block 2850, all STAs may contend for access, if a contention periodis defined for the legacy system. This allows legacy systems, not ableto communicate during the contention free period, to make requestsand/or attempt to transmit. In decision block 2860, the process maycontinue by returning to block 2810, or may stop.

FIG. 29 illustrates the combination of legacy and new class media accesscontrol. A legacy MAC protocol 2910 is shown above a new class protocol2930, which, when combined, form a MAC protocol such as combined MACprotocol 2950. In this example, 802.11 legacy signaling is used forillustration purposes. Those of skill in the art will realize thetechniques disclosed herein may be applied to any of a variety of legacysystems, and any new class MAC protocol, including any combination ofthe features disclosed herein.

Legacy MAC protocol 2910 comprises beacons 2902, which identify theBeacon interval. The legacy Beacon interval comprises contention freeperiod 2904 followed by contention period 2906. Various contention freepolls 2908A-N may be generated during the contention free period 2904.The contention free period 2904 is terminated by contention free periodend 2910. Each beacon 2902 is transmitted at Target Beacon TransmissionTime (TBTT) in 802.11 example embodiments. New class MAC protocol 2930comprises MAC frames 2932A-N.

The combined Beacon interval 2950 illustrates the interoperability oflegacy and new class MAC protocols during the contention free period2904. New class TDD MAC frame intervals 2932 are included followed bylegacy polls CF poll 2908A-N. The contention free period terminates withCFPEND 2910, followed by a contention period 2906. New class TDD MACframe intervals 2932 may be any type optionally including variousaspects detailed herein. In an example embodiment, new class TDD MACframe interval 2932 comprises various segments such as those illustratedwith respect to FIG. 20 above. Thus, a new class TDD MAC frame interval,in this example, comprises pilot 510, a control channel 520, a forwardtransmit channel 530, ad hoc peer-peer section (A-TCH) 2010, a reverselink transmit channel 540, and a random access channel 550.

Note that, during the CFP 2904, legacy STAs should not interfere withany new class WLAN transmission. The AP may poll any legacy STA duringthe CFP, permitting mixed mode operation in the segment. In addition,the AP may reserve the entire CFP 2904 for new class usage and push alllegacy traffic to the contention period (CP) 2906 near the end of theBeacon interval.

The example 802.11 legacy standard requires the CP 2906 be long enoughto support an exchange between two legacy terminals. Thus, the beaconmay be delayed, resulting in time jitter in the system. If desired, tomitigate jitter, the CFP interval may be shortened to maintain a fixedbeacon interval. Timers used to establish the CFP and CP may be set suchthat the CFP is long (i.e., around 1.024 sec) relative to the CP (i.e.,less than 10 msec). However, if, during the CFP, the AP polls legacyterminals, the duration of their transmission may be unknown and maycause additional time jitter. As a result, care must be taken tomaintain QoS for new class STAs when accommodating legacy STAs on thesame FA. The legacy 802.11 standard synchronizes to Time Units (TU) of1.024 msec. The new class MAC may be designed to be synchronous with alegacy system, employing a MAC frame duration of 2 TUs or 2.048 msec, inthis example.

In some embodiments, it may be desirable to insure that the new classMAC frame be made synchronous. That is, the MAC frame clock for thesystem may be continuous and that MAC frame boundaries, whentransmitted, start on multiples of the 2.048 msec frame interval. Inthis way, sleep mode for STAs may be easily maintained.

New class transmissions do not need to be compatible with legacytransmissions. The headers, preambles, etc., may all be unique to thenew class system, examples of which are detailed throughout thisspecification. Legacy STAs may attempt to demodulate these, but willfail to decode properly. Legacy STAs in sleep mode will generally not beaffected.

FIG. 30 depicts method 3000 for earning a transmit opportunity. Method3000 may be deployed as block 2830 in an example embodiment of method2800, illustrated above. The process begins with decision block 3010, inwhich an access may be scheduled or unscheduled. Those of skill in theart will recognize that, while this example illustrates two types ofaccess, in any given embodiment either one or both of these access typesmay be supported. In decision block 3010, if unscheduled access isdesired, proceed to block 3040 to contend for access. Any number of thecontention based access techniques may be deployed. Once a transmissionopportunity (TXOP) has been earned, transmit according to the transmitopportunity in block 3050. Then the process may stop.

In block 3010, if scheduled access is desired, proceed to block 3020 torequest access. This access request may be made on a random accesschannel, during ad hoc contention, or any of the other techniquesdisclosed herein. In block 3030, when the access request is granted, anallocation will be received. Proceed to block 3050 to transmit the TXOPaccording to the received allocation.

In some instances, it may be desirable to accommodate interoperationbetween a new class AP, and its associated BSS, with an overlappinglegacy BSS, in the same frequency allocation. The legacy BSS may beoperating in DCF or PCF/HCF mode, and so synchronization between the newclass BSS and legacy BSS may not always be achievable.

If the legacy BSS is operating in PCF or HCF mode, the new class AP mayattempt to synchronize to the TBTT. If this is possible, the new classAP may seize the channel during the contention period, using any ofvarious mechanisms, examples of which are described above, to operatewithin the overlapped BSS area. If the legacy BSS is operating underDCF, the new class AP may also attempt to seize the channel and announcea CFP to clear the channel.

There may be situations where some or all of the STAs in the legacy BSSdo not receive the new class AP transmissions. In this case, thoselegacy STAs may interfere with operation of the new class WLAN. To avoidthis interference, the new class stations may default to CSMA-basedoperation and rely on peer-peer transmissions (this is detailed furtherbelow with respect to FIGS. 33-34).

FIG. 31 depicts example method 3100 for sharing a single FA withmultiple BSSs. In block 3110, a legacy access point transmits a beacon.A new class access point, sharing the same frequency assignment, maysynch to the TBTT associated with the beacon (optional). In block 3120,if a legacy contention free period has been prescribed according to thebeacon, it is carried out. Once the contention free period, if any, iscomplete, then all STAs may contend for access during a prescribedcontention period. In block 3130, the new class access point contendsfor access during the contention period. In block 3140, new class STAsmay access the shared medium during the period for which the new classaccess point has contended for access. The types of access during thisnew class access may include any of the aspects detailed herein. Avariety of techniques may be used, such as those detailed above, toindicate to legacy STAs the amount of time for which the access point isreserving the channel. Once this period has completed, then legacy STAsmay contend in block 3150. In decision block 3160 the process maycontinue by returning to block 3110 or may stop.

FIG. 32 illustrates overlapping BSSs using a single FA. Legacy system3210 transmits beacons 3205 (3205A and 3205B are shown illustrating theTBTT and the overall Beacon interval of the legacy system). Beacon 3205Aidentifies contention free period 3210 and contention period 3215.During contention free period 3210, legacy contention free polls 3220A-Nmay be carried out followed by the indicator of the end of thecontention free period 3225.

Stations in new class WLAN 3240 monitor the channel, receive beacon3205, and refrain from accessing media until an opportunity to contendfor access arrives. In this example, the earliest opportunity is duringthe contention free period. After PIFS 3230, the new class access pointtransmits a legacy signal 3245 to indicate to legacy stations the amountof time that the channel will be occupied. A variety of symbols may beused to perform this function, examples of which have been detailedabove. Various other signals may be deployed depending on the legacysystem with which interoperability is desired. Legacy STAs withinreception range of legacy signal 3245 may avoid accessing a channeluntil the end of new class access period 3250. Period 3250 comprises oneor more TDD MAC frame intervals 3260 (3260A-N, in this example). TDD MACframe intervals 3260 may be any type, examples of which comprise one ormore of the aspects detailed herein.

In an example embodiment, the new class AP seizes the channel at timedintervals (i.e., every 40 msec the new class AP seizes the channel for20 msec). The new class AP may maintain a timer to insure it is onlyholding the channel for a desired duration, thereby guaranteeing fairsharing of the channel. In seizing the channel, the new class AP may usevarious signaling techniques. For example, CTS/RTS or a legacy beaconannouncing a new CFP may be transmitted.

During the new class interval 3250, an example first TDD MAC frameinterval may be defined as follows: First, send a beacon plus F-CCHindicating the UTs on the list to be polled in the current MAC frame.After the F-CCH, broadcast a stretch of MIMO pilot to allow the STAs toacquire and form an accurate measure of the MIMO channel. In an exampleembodiment, excellent performance may be achieved with 2 short OFDMsymbols per antenna. This implies that the F-TCH in the initial MACframe may be composed of roughly 8 MIMO pilot symbols. The R-TCH portionof the first MAC frame may be structured such that STAs on the poll listtransmit steered MIMO pilot and a rate indicator (for the downlink) withacknowledgement back to the AP. At this point, in this example, allterminals on the poll list are ready to operate in a normal scheduledmanner in the next TDD MAC frame interval. The TDD MAC frame intervalsfollowing the first TDD MAC frame interval may then be used to exchangedata, coordinated by the AP, using any of the techniques disclosedherein.

As mentioned above, new class stations may default to CSMA-basedoperation and rely on peer-peer transmissions in certain situations (forexample, situations when some or all of the STAs in the legacy BSS donot receive the new class AP transmissions). In such cases, the On/Offcycling described above might not be advantageous, or even possible. Inthese cases, new class stations may default to peer-peer operation.

FIG. 33 depicts example method 3300 for performing high-speed peer-peercommunication, using various techniques disclosed herein, whileinteroperating with a legacy BSS. The process begins in block 3310,where a first STA having data to send to a second STA contends foraccess. In block 3320, having contended for access successfully, thestation clears the medium using a legacy signal, such as those describedabove. In block 3330, the first STA transmits a request (along with apilot) to a second STA. The second STA is able to measure the channelaccording to the pilot transmitted. The second STA transmits channelfeedback to the first STA. Thus, in block 3340 the first stationreceives a response with channel feedback (rate feedback, for example).In block 3350 the first STA transmits the pilot and steered data to thesecond station according to the feedback. In block 3360 the second STAmay transmit to the first STA acknowledgement, and may transmitcontinued rate feedback for use in further transmission. The legacysignal used to clear the medium allows blocks 3330 to 3360 to be carriedout using any of the high-speed techniques and improvements to legacysystems such as those disclosed herein. Once a STA has cleared themedium, any peer-peer MAC protocol may be deployed within the scope ofthe present invention period. The process may continue as depicted indecision block 3370 by returning to block 3310, or the process may stop.

In an example embodiment, with peer-peer mode, seizing the channel worksaccording to the legacy rules for CSMA. In this example, PCF and HCF arenot employed, and there may not necessarily be a centralized networkarchitecture. When a new class STA wishes to communicate with anothernew class STA (or AP), the STA seizes the channel. The firsttransmission consists of sufficient MIMO pilot plus some messagerequesting a connection to be established. CTS and RTS may be employedto clear out the area and reserve time. The requesting STAs message mustcontain the STAs BSS ID, the STAs MAC ID, and the target STAs MAC ID (ifknown). The response should contain the BSS ID of the responding STA.This allows the STAs to determine whether they need to perform receivercorrection of transmit steering vectors, if steering is used. Note thattransmit steering does not have to be used in this case, although it maybe advantageous to do so if the STAs have all calibrated with adesignated AP coordinating the BSS.

As described with respect to FIG. 33, a response may contain MIMO pilot(steered, if employed) plus some indication of rate. Once this exchangehas occurred, steering is possible on each link. However, if the STAsbelong to different BSSs, the first steered transmission between the STAthat initiated the connection may contain steered MIMO pilot to allowthe responding STA's receiver to correct for the phase differentialbetween the different BSSs.

In this example embodiment, once the initial exchanges have occurred,steering is possible. The exchanges should adhere to the SIFS intervalbetween downlink and uplink transmissions. Because of potentialprocessing delays in computing eigenvectors for steering, this mayrequire that the STAs use Minimum Mean Squared Error (MMSE) processinginstead of eigenvector processing. Once the steering vectors arecomputed, STAs may start to use the eigenvectors on the transmit sideand the receive side may continue to employ MMSE processing, adaptingtoward the optimal spatial matched filter solution. Tracking and ratecontrol may be facilitated by periodic feedback between the two STAs.The SIFS interval may be adhered to in order for the STAs to maintaincontrol over the channel.

FIG. 34 illustrates peer-peer communication using MIMO techniques bycontending for access (i.e. unmanaged) on a legacy BSS. In this example,initiating station 106A contends for access on the channel. When it hassuccessfully seized the channel, MIMO pilot 3405 is transmitted,followed by request 3410. The message may contain the BSS ID, theinitiating STA's MAC ID and a target STA's MAC ID, if known. Othersignaling may be used to further clear the channel, such as CTS and RTS.The responding STA 106B transmits steered pilot 3420 followed byacknowledgement and rate feedback 3425. Steered pilot 3420 istransmitted SIFS 3415 following request 3410. In the example embodiment,in which the legacy access point is an 802.11 access point, recall thatSIFS is the highest priority and, thus, the responding station 106B willretain control of the channel. The various transmissions detailed inFIG. 34 may be transmitted SIFS apart from each other to maintaincontrol of the channel until the peer-peer communication is complete.

In an example embodiment, a maximum duration for channel occupation maybe determined. Steered pilot 3430, subsequent to rate feedback 3425, anddata 3435 are transmitted from the initiating STA 106A to the respondingSTA 106B in accordance with that rate feedback. Following data 3435, theresponding STA 106B transmits steered pilot 3440 and acknowledgement andrate control 3445. In response, initiating station 106A transmitssteered pilot 3450 followed by data 3455.

The process may continue indefinitely or up to the maximum time allowedfor channel access, depending on the deployment period. Not shown inFIG. 34, the responding STA may also transmit data and the initiatingstation may transmit rate control as well. These data segments may becombined with those shown at FIG. 34 to maximize efficiency (i.e., SIFSneed not be interjected between these transmissions).

When two or more BSSs overlap, it may desirable to deploy mechanismsthat allow the channel to be shared in a coordinated manner. Severalexample mechanisms are outlined below, along with example operatingprocedures associated with each. These mechanisms may be deployed incombination.

A first example mechanism is Dynamic Frequency Selection (DFS). Beforeestablishing a BSS, WLANs may be required to search the wireless mediumto determine the best Frequency Allocation (FA) to establish operationsfor the BSS. In the process of searching the candidate FA's, an AP mayalso create a neighbor list to facilitate redirection and inter-APhandoff. In addition, the WLAN may synchronize MAC frame timing withneighbor BSSs (described further below). DFS may be used to distributeBSSs to minimize the need for inter-BSS synchronization.

A second example mechanism is inter-BSS Synchronization. During a DFSprocedure, an AP may acquire the timing of the neighbor BSSs. Ingeneral, it may be desirable to synchronize all BSSs (on a single FA inone embodiment, or across multiple FAs in an alternate embodiment) tofacilitate inter-BSS handoff. However, with this mechanism, at leastthose BSSs operating on the same FA in close proximity to each othersynchronize their MAC frames. In addition, if co-channel BSSs areoverlapping (i.e. the APs can hear each other), the newly arriving APmay alert the established AP of its presence and institute a resourcesharing protocol, as follows.

A third example mechanism is a resource sharing protocol. OverlappingBSSs on the same FA may equitably share the channel. This may be done byalternating MAC frames between BSSs in some defined fashion. This allowstraffic in each BSS to use the channel without risking interference fromneighbor BSSs. The sharing may be done between all overlapping BSSs. Forexample, with 2 overlapping BSSs, one AP uses even numbered MAC framesand the other AP uses odd numbered MAC frames. With 3 overlapping BSSs,the sharing may be performed modulo-3, etc. Alternate embodiments maydeploy any type of sharing scheme. Control fields in the BCH overheadmessage may indicate if resource sharing is enabled and the type ofsharing cycles. In this example, timing for all STAs in the BSS adjustto the appropriate sharing cycle. In this example, latency will beincreased with overlapping BSSs.

A fourth example mechanism is STA assisted re-synchronization. It ispossible that two BSSs do not hear each other, but a new STA in theoverlapped area can hear both. The STA can determine the timing of bothBSSs and report this to both. In addition, the STA can determine thetime offset and indicate which AP should slip its frame timing and byhow much. This information has to be propagated to all BSSs connected tothe AP and they all have to re-establish frame timing to achievesynchronization. Frame resynchronization can be announced in the BCH.The algorithm can be generalized to handle more unaware overlappingBSSs.

Example procedures are detailed below, which may be deployed in one ormore of the mechanisms just described.

Synchronization may be performed by AP's on power-up, or at otherdesignated times. System timing may be determined by searching all FA'sfor nearby systems. To facilitate synchronization, a set of orthogonalcodes may be used to aid in discriminating different APs. For example,APs have known beacons repeated every MAC frame. These beacons may becovered with Walsh sequences (e.g. of length 16). Thus a device, such asan AP or STA, may perform Pilot Strength Measurements (PSMs) of thelocal APs to determine the overlapping BSSs. Detailed further below,active STAs, associated with an AP, may transmit echoes to assist insynchronization. The echoes may use timing and covering corresponding tothe AP cover. Thus, when BSSs overlap, but the respective APs for thoseBSSs may not be able to detect signals from each other, a STA echo maybe receivable by a neighbor AP, thus providing information about its AP,and a signal with which the neighbor AP may synchronize. Note thatorthogonal cover codes may be reused on different FAs.

Selection of a Walsh cover may be done deterministically based on theset of undetected Walsh covers (i.e., select a Walsh cover that is notdetected on a neighboring AP). If all covers are present, the codecorresponding to the weakest Received Signal Level (RSL) may be re-usedby the new AP. Otherwise, in one embodiment, the code may be selectedthat maximizes the operating point for the AP (see structured powerbackoff for adaptive reuse, detailed below).

In this example, frame counters transmitted by each AP are staggeredrelative to each other. The stagger employed corresponds to the Walshcover index. Thus, AP0 uses Walsh code 0. APj uses Walsh cover j, andhas its frame counter equal to 0 whenever the AP0 frame counter=j.

On power-up, or at any time synchronization is to be performed, an APlistens for neighbor AP beacons and/or STA echoes. Upon no detection ofneighbor systems, the AP establishes its own time reference. This can bearbitrary, or related to GPS, or any other local time reference. Upondetection of a single system, the local timing is establishedaccordingly. If the AP detects two or more systems operating withdifferent time lines, the AP may synchronize with system having thestrongest signal. If the systems are operating on the same frequencyassignment (FA), the AP may attempt to associate with the weaker AP toinform it of the other nearby AP operating on an independent clock. Thenew AP attempts to inform the weaker AP of the timing skew required tosynchronize both AP zones. The weaker zone AP may then skew its timing.This may be repeated for multiple neighbor APs. The new AP can establishits timing with the synchronized timing of the two or more systems. In asituation where all neighbor APs are unable, for whatever reason, tosynchronize to a single timing, the new AP may synchronize to any of theneighboring APs.

Dynamic frequency selection may be performed by AP's on power-up. Asstated above, it is typically desirable to minimize BSS overlap with DFSselection, to minimize the number of BSSs requiring synchronization, andany delay or throughput reduction that may be associated withsynchronization (i.e., a BSS with access to the entire medium on an FAmay be more efficient than a BSS which must share the medium with one ormore neighboring BSSs). After synchronization, the new AP may select theFA that has the minimum RSL associated with it (i.e. when measuringneighbor APs, or during the echo period). Periodically, the AP may querythe STAs for AP pilot measurements. Similarly, the AP may schedulesilent periods to enable assessment of the interference levels at the APcaused by STAs from other zones (i.e. neighboring BSSs). If the RSLlevels are excessive, the AP may attempt to find another FA duringunscheduled periods, and/or institute a power backoff policy, asdescribed below.

As described above, APs may be organized according to a pilot covercode. Each AP may use a Walsh sequence cover of length 16, in thisexample. Any number of codes of various lengths may be deployed. Thepilot cover is used to modulate the sign of the beacon over asuper-frame period. In this example, the super-frame period isequivalent to 32 ms (i.e. 16 consecutive MAC frame beacons). STAs maythen coherently integrate over the superframe interval to determine thepilot power associated with a given AP. As above, an AP may select itsWalsh code from the pool of undetected Walsh codes available. If allcodes are detected (on the same FA), then the AP may rank these in orderof strongest to weakest. The AP may re-use the Walsh code thatcorresponds to the weakest detected Walsh code.

To facilitate identification of neighbor APs, STAs may be used totransmit an echo to identify their respective AP. Thus, as describedabove, an AP that doesn't detect a neighbor AP may detect acorresponding STA echo, thus identifying the AP and its timing. Each APmay transmit configuration information in its beacon, and each STA mayoperate as a repeater to retransmit the AP configuration information, aswell as timing, to any receiving neighbor AP.

Active STAs may be required to transmit, upon command from the AP, apredefined pattern that allows nearby APs operating on the same FA todetect the presence of the neighbor system. A simple way to facilitatethis is to define an observation interval in the MAC frame (e.g. betweenthe FCH and RCH segments) that is not used by the AP for any traffic.The duration of the observation interval may be defined to be longenough to handle the maximum differential propagation delay between STAsassociated with the AP and STAs associated with a neighbor AP (e.g. 160chips or 2 OFDM symbols). For example, STAs associated with the AP usingWalsh cover code j may transmit the echo whenever its Mac framecounter=0. The echo is coded with information necessary to allowneighbor APs to detect the presence and efficiently co-exist with STAsin the adjacent AP zone.

Structured power backoff for adaptive reuse may be deployed. When asystem becomes congested to the point where each FA must be reused inthe vicinity of another AP, it may be desirable to impose a structuredpower backoff scheme to allow terminals in both zones to operate atmaximum efficiency. When congestion is detected, power control can beused to improve the system's efficiency. That is, instead oftransmitting at full power all of the time, the APs may use a structuredpower back-off scheme that is synchronized with their MAC frame counter.

As an example, suppose that two APs are operating on the same FA. Oncethe APs detect this condition, they may institute a known power backoffpolicy. For example, both APs use a backoff scheme that permits fullpower, Ptot, on MAC frame 0, Ptot(15/16) on MAC frame 1, . . . Ptot/16on MAC frame 15. Since the APs are synchronized, and their framecounters staggered, neither AP zone is using full power simultaneously.The objective is to select the backoff pattern that allows STAs in eachAP zone to operate at the highest possible throughput.

The backoff pattern used by a given AP may be a function of the degreeof interference detected. In this example, up to 16 known backoffpatterns may be used by a given AP. The backoff pattern used may beconveyed by the APs in the BCH and in the echoes transmitted by STAsassociated with an AP.

An example backoff scheme is detailed in U.S. Pat. No. 6,493,331,entitled “Method and apparatus for controlling transmissions of acommunications systems,” by Walton et. al, assigned to the assignee ofthe present invention.

Another example embodiment of a technique for interoperability withlegacy systems is depicted in FIG. 53. An example MAC frame 1500 isshown, as detailed above with respect to FIG. 15. A slotted mode isintroduced in which slot intervals 5310 are defined. A slot interval5310 comprises a MIMO pilot interval 5315 and slot gap 5320. Pilots 5315are inserted, as shown, to reserve the channel from interference byother stations (including APs) that operate according to rules, such asEDCA. Modified MAC frame 5330 comprises substantially the MAC frame 1500with pilots 5315 inserted to retain control of the medium. FIG. 53 isillustrative only, as will be evident to one of skill in the art. Aslotted mode may be incorporated with any type of MAC frame, variousexamples of which are detailed herein.

In this example, for purposes of illustration, assume a legacy 802.11system that uses MAC frames that are multiples of 1.204 ms. The MACframe may be set to be 2.048 ms to be synchronous. At the Target BeaconTransmit Time (TBTT), an announce CFP duration to get STAs to set theirNAV's. During the CFP, STAs in the BSS should not transmit unlesspolled. Optionally, as described previously, an AP may send out an RTSand have STAs echo an identical CTS to clear out the BSS further. ThisCTS may be a synchronized transmission from all the STAs. In thisexample, jitter may be eliminated by insuring MAC frames always start on2.048 ms boundaries. This maintains time synch betweenadjacent/overlapping BSSs even with foreshortened TBTTs. Various othertechniques, such as those described above, may be combined with thetechnique described below. Once the medium is reserved for modified MACframe 5330, using any available technique, slotted mode may be deployedto maintain possession of the medium, to prevent a legacy STA frominterfering with the scheduled transmissions, thus potentially reducingthroughput gains of a new class system (i.e. one using a scheme such asshown in FIG. 15 or FIG. 53, or various others detailed herein).

In this example, the new class AP is subject to CSMA rules to seize thechannel. Prior to this however, it should attempt to determine thepresence of another BSS, either by listening for the beacon, or otherSTAs. Synchronization is not required, however, to permit fair resourcesharing.

Once the neighbor(s) BSS(s) has been detected, the new class AP canseize the channel by transmitting its beacon. To lock out other users,the new class AP transmits pilot with a frequency that prevents otherSTAs to use the channel (i.e. no idle periods any longer than PIFS=25usec).

The new class AP may set a timer that allows it to occupy the channelfor a fixed duration determined to be fair. This may be roughlysynchronized with the legacy AP's beacon period or asynchronous (i.e.100 msec every 200 msec).

The new class AP may seize the channel at any point during its permittedinterval, which can be delayed by legacy BSS users. The new class AP mayrelinquish the channel before its time has expired if there is notraffic to serve. When the new class AP seizes the channel, it have itsuse limited for an equitable period of time. Furthermore, the timingestablished by the new class AP may be consistent with the MAC frametiming established. That is, new class beacons occur on 2.048 msecboundaries of the new class AP clock. This way, new class STAs maymaintain synchronization by looking at these specific intervals todetermine if the HT AP has seized the channel.

The new class AP may announce its frame parameters in a beacon. Part ofthe frame parameters may include the pilot interval spacing indicatingthe frequency of pilot transmission throughout the MAC frame. Note thatthe new class AP may schedule STAs such that their transmission overlapsthe periodic burst pilot. In this case, the STA whose assignmentoverlaps knows this and ignores the pilot during that period. Other STAsdo not know this and therefore use a threshold detector to validatewhether the pilot was transmitted during the prescribed interval.

It is possible that a STA may transmit a pilot at the instant the AP issupposed to transmit, or that the AP is transmitting steered pilot to aSTA during this interval. To prevent other STAs from using this pilot,thus corrupting their channel estimates, the AP pilot may use Walshcovers that are orthogonal to common pilot Walsh covers. A structure forassigning Walsh covers may be deployed. For example, when STAs and APsuse different Walsh covers, the Walsh space may include 2N covers, withN covers reserved for APs, and the remainder for STAs associated with agiven AP using a cover that is coupled in a known manner with therespective AP's Walsh cover.

When the new class AP transmits an assignment to a STA, it is expectingthe STA to transmit to it during the prescribed interval. It is possiblethe STA fails to receive the assignment, in which case the channel couldgo unused for an interval longer than PIFS. To prevent this fromoccurring, the AP may sense the channel for t<SIFS and determine if itis occupied. If not, the AP may immediately seize the channel bytransmitting pilot, phased accordingly.

New class channel assignments may be slotted to intervals of SIFS (16usec). This way channel occupancy can be guaranteed to keep off legacyusers during the period of new class exclusive usage.

The RCH must be designed to accommodate interoperability since theduration of the RCH could exceed 16 usec. If the RCH cannot be easilyaccommodated in a given embodiment, the RCH may be allocated to work inthe legacy modes when the new class MAC does not have control of thechannel (i.e. coexist in legacy mode). The F-RCH may be accommodated bypermitting STAs to transmit access requests anytime following a pilottransmission (i.e. wait 4 usec and transmit for 8 usec), as illustratedin FIG. 53.

Example Embodiment: Enhanced 802.11 MIMO WLAN

Detailed below is an example embodiment illustrating various aspectsintroduced above, as well as additional aspects. In this example, anenhanced 802.11 WLAN using MIMO is illustrated. Various MAC enhancementsare detailed, as well as corresponding data and messaging structures foruse at the MAC layer and physical layer. Those of skill in the art willrecognize that only an illustrative subset of features of a WLAN aredisclosed, and will readily adapt the teaching herein to 802.11 legacysystem interoperability, as well as interoperability with various othersystems.

The example embodiment, detailed below, features interoperability withlegacy 802.11a, 802.11g STAs as well as with the 802.11e draft andanticipated final standard. The example embodiment comprises a MIMO OFDMAP, so named to distinguish from legacy APs. Due to backwardcompatibility, as detailed below, legacy STAs are able to associate witha MIMO OFDM AP. However, the MIMO OFDM AP may explicitly reject anassociation request from a legacy STA, if desired. DFS procedures maydirect the rejected STA to another AP that supports legacy operation(which may be a legacy AP or another MIMO OFDM AP).

MIMO OFDM STAs are able to associate with an 802.11a or 802.11g BSS orIndependent BSS (IBSS) where no AP is present. Thus, for such operation,such a STA will implement all the mandatory features of 802.11a, 802.11gas well as the anticipated final draft of 802.11e.

When legacy and MIMO OFDM STAs share the same RF channel, either in aBSS or an IBSS, various features are supported: The proposed MIMO OFDMPHY spectral mask is compatible with the existing 802.11a, 802.11gspectral mask so that no additional adjacent channel interference isintroduced to legacy STAs. The extended SIGNAL field in the PLCP Header(detailed below) is backward compatible with the SIGNAL field of legacy802.11. Unused RATE values in the legacy SIGNAL field are set to definenew PPDU types (detailed below). The Adaptive Coordination Function(ACF) (detailed below) permits arbitrary sharing of the medium betweenlegacy and MIMO OFDM STAs. Periods of 802.11e EDCA, 802.11e CAP and theSCAP (introduced below) may be arbitrarily interspersed in any Beaconinterval, as determined by the AP scheduler.

As described above, a high performance MAC is required to effectivelyleverage the high data rates enabled by the MIMO WLAN physical layer.Various attributes of this example MAC embodiment are detailed below.Following are several example attributes:

Adaptation of the PHY rates and transmission modes effectively exploitthe capacity of the MIMO channel.

Low latency service of the PHY provides low end-to-end delays to addressthe requirements of high throughput (e.g. multimedia) applications. Lowlatency operation may be achieved with contention-based MAC techniquesat low loads, or using centralized or distributed scheduling in heavilyloaded systems. Low latency provides many benefits. For example, lowlatency permits fast rate adaptation to maximize the physical layer datarate. Low latency permits inexpensive MAC implementation with smallbuffers, without stalling ARQ. Low latency also minimizes end-to-enddelay for multimedia and high throughput applications.

Another attribute is high MAC efficiency and low contention overhead. Incontention based MACs, at high data rates, the time occupied by usefultransmissions shrinks while an increasing fraction of the time is wastedin overhead, collisions and idle periods. Wasted time on the medium maybe reduced through scheduling, as well as through aggregation ofmultiple higher layer packets (e.g. IP datagrams) into a single MACframe. Aggregated frames may also be formed to minimize preamble andtraining overhead.

The high data rates enabled by the PHY permit simplified QoS handling.

The example MAC enhancements, detailed below, are designed to addressthe above performance criteria in a manner that is backward compatiblewith 802.11g and 802.11a. In addition, support for and improvement tofeatures that are included in the draft standard 802.11e, describedabove, including features such as TXOP and Direct Link Protocol (DLP),as well as the optional Block Ack mechanism.

In describing the example embodiments below, new terminology is used forsome concepts introduced above. A mapping for the new terminology isdetailed in Table 1.

TABLE 1 Terminology Mapping Earlier Terminology Mapping to NewTerminology Terms used in prior paragraphs Terms used in subsequentparagraphs MUX PDU or MPDU MAC Frame Partial MPDU MAC Frame Fragment MACPDU PPDU Broadcast channel message (BCH) SCHED message and Controlchannel message (CCH) Control channel message sub- CTRLJ segments of theSCHED channels message TDD MAC frame interval Scheduled Access Period(SCAP) F-TCH (Forward Traffic Channel) Scheduled AP-STA transmissionsR-TCH (Reverse Traffic Channel) Scheduled STA-AP or STA-STAtransmissions A-TCH (Ad-hoc peer-to-peer Protected EDCA or MIMO OFDMTraffic Channel) EDCA PCCH (Peer-to-Peer Control PLCP Header SIGNALfield Channel) RCH FRACHFlexible Frame Aggregation

In this example embodiment, flexible frame aggregation is facilitated.FIG. 35 depicts encapsulation of one or more MAC frames (or fragments)within an aggregated frame. Frame aggregation permits the encapsulationof one or more MAC frames (or fragments) 3510 within an aggregated frame3520, which may incorporate header compression, detailed below.Aggregated MAC frame 3520 forms PSDU 3530, which may be transmitted as asingle PPDU. The aggregated frame 3520 may contain encapsulated frames(or fragments) 3510 of type data, management or control. When privacy isenabled, the frame payload may be encrypted. The MAC frame header of anencrypted frame is transmitted “in the clear.”

This MAC-level frame aggregation, as just described, permitstransmission of frames with zero IFS or BIFS (Burst Interframe Spacing,detailed further below) to the same receiving STA. In certainapplications, it is desirable to permit the AP to transmit frames withzero IFS, or aggregated frames, to multiple receiving STAs. This ispermitted through the use of the SCHED frame, discussed below. The SCHEDframe defines the start time of multiple TXOPs. Preambles and IFS may beeliminated when the AP makes back-to-back transmissions to multiplereceiving STAs. This is referred to as PPDU aggregation to distinguishfrom MAC-level frame aggregation.

An example aggregated MAC frame transmission (i.e. a PPDU) starts with apreamble followed by the MIMO OFDM PLCP HEADER (including a SIGNALfield, which may comprise two fields, SIGNAL1 and SIGNAL2), followed byMIMO OFDM training symbols (if any). Example PPDU formats are detailedfurther below with respect to FIGS. 49-52. The aggregated MAC frameflexibly aggregates one or more encapsulated frames or fragments thatare to be transmitted to the same receiving STA. (The SCHED message,detailed below, permits aggregation of TXOPs from the AP to multiplereceiving STAs.) There is no restriction on the number of frames andfragments that may be aggregated. There may be a limit to the maximumsize of an aggregated frame that is established through negotiation.Typically, the first and last frames in the aggregated frame may befragments that are created for efficient packing. When severalencapsulated data frames are included within an aggregated frame, theMAC headers of the data and QoS data frames may be compressed, asdetailed below.

The transmitting MAC may attempt to minimize PHY and PLCP overheads andidle periods through the use of flexible frame aggregation. This may beaccomplished by aggregating frames to eliminate inter-frame spacing andPLCP headers, as well as flexible frame fragmentation, to fully occupythe available space in a TXOP. In one example technique, the MAC firstcomputes the number of octets to be provided to the PHY based on thecurrent data rate and the duration of the assigned or contention-basedTXOP. Complete and fragmented MAC frames may then be packed to occupythe entire TXOP.

If a complete frame cannot be accommodated in the remaining space in aTXOP, the MAC may fragment the next frame to occupy as much as possibleof the remaining octets in the TXOP. Frames may be fragmentedarbitrarily for efficient packing. In an example embodiment, thisarbitrary fragmentation is subject to the restriction of a maximum of 16fragments per frame. In alternate embodiments, this limitation may notbe required. Remaining fragment(s) of the MAC frame may be transmittedin a subsequent TXOP. In the subsequent TXOP, the MAC may give higherpriority to fragments of an incompletely transmitted frame, if desired.

An Aggregation Header (2 octets, in this example), described furtherbelow, is inserted in the MAC Header of each encapsulated frame (orfragment) that is inserted in the aggregated frame. A Length field inthe Aggregation Header indicates the length (in octets) of theencapsulated MAC frame, and is used by the receiver to extract frames(and fragments) from the aggregated frame. The PPDU Size field in theproposed SIGNAL field provides the size of the MIMO OFDM PPDUtransmission (number of OFDM symbols) while the length of eachencapsulated MAC frame (in octets) is indicated by the AggregationHeader.

Header Compression of Encapsulated Frames

FIG. 36 depicts a legacy MAC frame 3600, comprising MAC Header 3660,followed by a frame body 3650 (which may include a variable number ofoctets, N) and a Frame Check Symbol (FCS) 3655 (4 octets, in thisexample). This prior art MAC frame format is detailed in 802.11e. MACHeader 3660 comprises a frame control field 3610 (2 octets), aduration/ID field 3615 (2 octets), a sequence control field 3635 (2octets), and a QoS control field 3645 (2 octets). In addition, fouraddress fields, Address 1 3620, Address 2 3625, Address 3, 3630, andAddress 4 3640 (6 octets each), are included. These addresses may alsobe referred to as TA, RA, SA, and DA, respectively. The TA is thetransmitting station address. The RA is the receiving station address.The SA is the source station address. The DA is the destination stationaddress.

When several encapsulated data frames are included within an aggregatedframe, the MAC headers of the data and QoS data frames may becompressed. Example compressed MAC headers for QoS data frames are shownin FIGS. 37-39. Note that the FCS is computed on the compressed MACheader and the (encrypted or unencrypted) payload.

As shown in FIG. 37-39, when frames are transmitted using a MIMO DataPPDU (Type 0000), an aggregation header field is introduced into the MACHeader 3660 of the MAC frame 3600 to create an encapsulated MAC frame,i.e. 3705, 3805, or 3905, respectively. The MAC Header, including theAggregation Header field, is called the Extended MAC Header (i.e. 3700,3800, or 3900). One or more encapsulated management, control and/or dataframes (including QoS data) may be aggregated into an aggregated MACframe. When data privacy is in use, the payload of the data or QoS dataframes may be encrypted.

The Aggregation Header 3710 is inserted for each frame (or fragment)inserted in the aggregated frame (3705, 3805, or 3905, respectively).Header compression is indicated by the Aggregation Header type field,detailed below. Frame headers of data and QoS data frames may becompressed to eliminate redundant fields. Aggregated frame 3705,depicted in FIG. 37, illustrates an uncompressed frame, which includesall four addresses and the Duration/ID field.

After an uncompressed aggregated frame is transmitted, additionalaggregated frames need not identify the transmitting and receivingstation addresses, as they are identical. Thus, Address 1 3620 andAddress 2 3625 may be omitted. The Duration/ID field 3615 does not needto be included for subsequent frames in the aggregated frame. Durationmay be used to set the NAV. The Duration/ID field is overloaded based oncontext. In Poll messages, it contains the Access ID (AID). In othermessages, the same field specifies the duration to set the NAV. Thecorresponding frame 3805 is illustrated in FIG. 38.

Further compression is available when the source address and destinationstation addresses contain duplicate information. In this case, Address 33630 and Address 4 3640 may also be removed, resulting in the frame 3905illustrated in FIG. 39.

When fields are removed, to decompress, the receiver may insert thecorresponding field from the previous header (after decompression) inthe aggregated frame. In this example, the first frame in an aggregatedframe always uses the uncompressed header. Decryption of the payload mayrequire some fields from the MAC Header that may have been removed forheader compression. After decompression of the frame header, thesefields may be made available to the decryption engine. The Length fieldis used by the receiver to extract frames (and fragments) from theaggregated frame. The Length field indicates the length of the framewith the compressed header (in octets).

After extraction, the Aggregation header field is removed. Thedecompressed frame is then passed to the decryption engine. Fields inthe (decompressed) MAC headers may be required for message integrityverification during decryption.

FIG. 40 illustrates an example Aggregation Header 3710. The AggregationHeader field is added to each frame (or fragment) header for one or moreframes (encrypted or un-encrypted) that are transmitted in a MIMO DataPPDU. The Aggregation Header comprises a 2 bit Aggregation Header Typefield 4010 (to indicate whether or not header compression is employed,and which type) and a 12 bit Length field 4030. Type 00 frames do notemploy header compression. Type 01 frames have the Duration/ID, Address1 and Address 2 fields removed. Type 10 frames have the same removedfields as type 01 frames, with the Address 3 and Address 4 fields alsoremoved. The Length field 4030 in the Aggregation Header indicates thelength of the frame in octets with the compressed header. 2 bits 4020are reserved. The Aggregation Header types are summarized in Table 2.

TABLE 2 Aggregation Header Type Bit 0 Bit 1 Meaning 0 0 Uncompressed 0 1Duration/ID, Address 1 and Address 2 fields are removed 1 0 Duration/ID,Address 1, Address 2, Address 3 and Address 4 fields are removed 1 1Reserved

In this example embodiment, all management and control frames that areencapsulated in an aggregated frame use the uncompressed frame headerwith Aggregation Header type 00. The following management frames may beencapsulated along with data frames in an aggregated frame: associationrequest, association response, reassociation request, reassociationresponse, probe request, probe response, disassociation, authentication,and deauthentication. The following control frames may be encapsulatedalong with data frames in an aggregated frame: BlockAck andBlockAckRequest. In alternate embodiments, any type of frames may beencapsulated.

Adaptive Coordination Function

The Adaptive Coordination Function (ACF) is an extension of the HCCA andEDCA that permits flexible, highly efficient, low latency scheduledoperation suitable for operation with the high data rates enabled by theMIMO PHY. FIG. 41 illustrates an example embodiment of a ScheduledAccess Period Frame (SCAP) for use in the ACF. Using a SCHED message4120, an AP may simultaneously schedule one or more AP-STA, STA-AP orSTA-STA TXOPs over the period known as the Scheduled Access Period 4130.These scheduled transmissions are identified as scheduled transmissions4140. The SCHED message 4120 is an alternative to the legacy HCCA Poll,detailed above. In the example embodiment, the maximum permitted valueof the SCAP is 4 ms.

Example scheduled transmissions 4140 are shown in FIG. 41 forillustration, including AP to STA transmissions 4142, STA to APtransmissions 4144, and STA to STA transmissions 4146. In this example,the AP transmits to STA B 4142A, then to STA D 4142B, and then to STA G4142C. Note that gaps need not be introduced between these TXOPs, as thesource (the AP) is the same for each transmission. Gaps are shownbetween TXOPs when the source changes (example gap spacings are detailedfurther below). In this illustration, after AP to STA transmissions4142, STA C transmits to the AP 4144A, then, after a gap, STA Gtransmits to the AP 4144B, and then, after a gap, STA E transmits to theAP 4144C. A peer to peer TXOP 4146 is then scheduled. In this case, STAE remains as the source (transmitting to STA F), so no gap needs to beintroduced if the STA E transmit power is unchanged, otherwise a BIFSgap may be used. Additional STA to STA transmissions may be scheduled,but are not shown in this example. Any combination of TXOPs may bescheduled, in any order. The order of TXOP types shown is an exampleconvention only. While it may be desirable to schedule TXOPs to minimizethe required number of gaps, it is not mandatory.

The Scheduled Access Period 4130 may also contain a FRACH Period 4150dedicated to Fast Random Access Channel (FRACH) transmissions (wherein aSTA may make a request for an allocation) and/or a MIMO OFDM EDCA 4160period where MIMO STAs may use EDCA procedures. These contention-basedaccess periods are protected by the NAV set for the SCAP. During theMIMO OFDM EDCA 4160 period, MIMO STAs use EDCA procedures to access themedium without having to contend with legacy STAs. Transmissions duringeither protected contention period use the MIMO PLCP header (detailedfurther below). The AP provides no TXOP scheduling during the protectedcontention period, in this embodiment.

When only MIMO STAs are present, the NAV for the SCAP may be set througha Duration field in the SCHED frame (the SCHED frame is detailed furtherbelow). Optionally, if protection from legacy STAs is desired, the APmay precede the SCHED frame 4120 with a CTS-to-Self 4110 to establishthe NAV for the SCAP at all STAs in the BSS.

In this embodiment, MIMO STAs obey the SCAP boundary. The last STA totransmit in a SCAP must terminate its TXOP at least PIFS duration beforethe end of the SCAP. MIMO STAs also obey the scheduled TXOP boundariesand complete their transmission prior to the end of the assigned TXOP.This allows the subsequent scheduled STA to start its TXOP withoutsensing the channel to be idle.

The SCHED message 4120 defines the schedule. Assignments of TXOPs(AP-STA, STA-AP and/or STA-STA) are included in the CTRLJ elements(4515-4530 in FIG. 45, detailed below) in the SCHED frame. The SCHEDmessage may also define the portion of the SCAP 4100 dedicated to FRACH4150, if any, and a protected portion for EDCA operation 4160, if any.If no scheduled TXOP assignments are included in the SCHED frame, thenthe entire SCAP is set aside for EDCA transmissions (including anyFRACH) protected from legacy STAs by the NAV set for the SCAP.

The maximum length of scheduled or contention-based TXOP permittedduring the SCAP may be indicated in an ACF capabilities element. In thisembodiment, the length of the SCAP does not change during a Beaconinterval. The length may be indicated in the ACF capabilities element.An example ACF element comprises a SCAP Length (10 bits), a Maximum SCAPTXOP Length (10 bits), a Guard IFS (GIFS) Duration (4 bits), and a FRACHRESPONSE (4 bits). The SCAP Length indicates the length of the SCAP forthe current Beacon interval. The field is encoded in units of 4 μs. TheMaximum SCAP TXOP Length indicates the maximum permissible TXOP lengthduring a SCAP. The field is encoded in units of 4 μs. GIFS Duration isthe guard interval between consecutive scheduled STA TXOPs. The field isencoded in units of 800 ns. FRACH RESPONSE is indicated in units ofSCAPs. The AP must respond to a request received using an FRACH PPDU byproviding the STA with a scheduled TXOP within FRACH RESPONSE SCAPs.

FIG. 42 shows an example of how the SCAP may be used in conjunction withHCCA and EDCA. In any Beacon interval (illustrated with beacons4210A-C), the AP has complete flexibility to adaptively intersperseduration of EDCA contention-based access with the 802.11e CAP and theMIMO OFDM SCAP.

Thus, using the ACF, the AP may operate as in HCCA, but with theadditional capability of allocating periods for SCAP. For example, theAP may use CFP and CP as in the PCF, allocate a CAP for polled operationas in HCCA, or may allocate a SCAP for scheduled operation. As shown inFIG. 42, in a Beacon interval, the AP may use any combination of periodsfor contention based access (EDCA) 4220A-F, CAP 4230A-F, and SCAP4100A-I. (For simplicity, the example in FIG. 42 does not show any CFP.)The AP adapts the proportion of the medium occupied by different typesof access mechanisms based on its scheduling algorithms and itsobservations of medium occupancy. Any scheduling technique may bedeployed. The AP determines whether admitted QoS flows are beingsatisfied and may use other observations including measured occupancy ofthe medium for adaptation.

HCCA and associated CAPs are described above. An illustrative exampleCAP 4230 is shown in FIG. 42. An AP TXOP 4232 is followed by a Poll4234A. HCCA TXOP 4236A follows Poll 4234A. Another Poll 4234B istransmitted, followed by another respective HCCA TXOP 4236B.

EDCA is described above. An illustrative example EDCA 4220 is shown inFIG. 42. Various EDCA TXOPs 4222A-C are shown. A CFP is omitted in thisexample.

A SCAP 4100, as shown in FIG. 42, may be of the format detailed in FIG.41, including an optional CTS to Self 4110, SCHED 4120, and ScheduledAccess Period 4130.

The AP indicates scheduled operation using the 802.11 Delivery TrafficIndication Message (DTIM) message as follows. The DTIM contains a bitmapof Access IDs (AIDs) for which the AP or another STA in the BSS hasbacklogged data. Using the DTIM, all MIMO-capable STAs are signaled tostay awake following the Beacon. In a BSS where both legacy and MIMOSTAs are present, legacy STAs are scheduled first, immediately followingthe Beacon. Right after the legacy transmissions, the SCHED message istransmitted that indicates the composition of the Scheduled AccessPeriod. MIMO-capable STAs not scheduled in a particular Scheduled AccessPeriod may sleep for the remainder of the SCAP and wake up to listen forsubsequent SCHED messages.

Various other modes of operation are enabled with ACF. FIG. 43 shows anexample operation where each Beacon interval comprises a number of SCAPs4100 interspersed with contention-based access periods 4220. This modepermits “fair” sharing of the medium where MIMO QoS flows are scheduledduring the SCAP while MIMO non-QoS flows use the contention periodsalong with legacy STAs, if present. Interspersed periods permit lowlatency service for MIMO and legacy STAs.

As described above, the SCHED message in the SCAP may be preceded by aCTS-to-Self for protection from legacy STAs. If no legacy STAs arepresent, CTS-to-Self (or other legacy clearing signal) is not required.The Beacon 4210 may set a long CFP to protect all SCAPs from anyarriving legacy STAs. A CP at the end of the Beacon interval allowsnewly arriving legacy STAs to access the medium.

Optimized low-latency operation with a large number of MIMO STAs may beenabled using the example operation shown in FIG. 44. In this example,the assumption is that legacy STAs, if present, require only limitedresources. The AP transmits a Beacon, establishing a long CFP 4410 and ashort CP 4420. A Beacon 4210 is followed by any broadcast/multicastmessages for legacy STAs. Then SCAPs 4100 are scheduled back-to-back.This mode of operation also provides optimized power management, as theSTAs need to awake periodically to listen to SCHED messages and maysleep for the SCAP interval if not scheduled in the current SCAP.

Protected contention-based access for MIMO STAs is provided through theFRACH or MIMO EDCA periods included in the Scheduled Access Period 4130of the SCAP 4100. Legacy STAs may obtain contention-based access to themedium during the CP 4420.

Consecutive scheduled transmissions from the AP may be scheduledimmediately following transmission of the SCHED frame. The SCHED framemay be transmitted with a preamble. Subsequent scheduled APtransmissions may be transmitted without a preamble (an indicator ofwhether or not a preamble is included may be transmitted). An examplePLCP preamble is detailed further below. Scheduled STA transmissionswill begin with a preamble in the example embodiment.

Error Recovery

The AP may use various procedures for recovery from SCHED receiveerrors. For example, if a STA is unable to decode a SCHED message, itwill not be able to utilize its TXOP. If a scheduled TXOP does not beginat the assigned start time, the AP may initiate recovery by transmittingat a PIFS after the start of the unused scheduled TXOP. The AP may usethe period of the unused scheduled TXOP as a CAP. During the CAP, the APmay transmit to one or more STAs or Poll a STA. The Poll may be to theSTA that missed the scheduled TXOP or another STA. The CAP is terminatedprior to the next scheduled TXOP.

The same procedures may also be used when a scheduled TXOP terminatesearly. The AP may initiate recovery by transmitting at a PIFS after theend of the last transmission in the scheduled TXOP. The AP may use theunused period of a scheduled TXOP as a CAP, as just described.

Protected Contention

As described above, a SCAP may also contain a portion dedicated to FRACHtransmissions and/or a portion where MIMO STAs may use EDCA procedures.These contention-based access periods may be protected by the NAV setfor the SCAP.

Protected contention complements low latency scheduled operation bypermitting STAs to indicate TXOP requests to assist the AP inscheduling. In the protected EDCA period, MIMO OFDM STAs may transmitframes using EDCA based access (protected from contention with legacySTAs). Using legacy techniques, STAs may indicate TXOP duration requestor buffer status in the 802.11e QoS Control field in the MAC Header.However, the FRACH is a more efficient means of providing the samefunction. During the FRACH period, STAs may use slotted Aloha likecontention to access the channel in fixed size FRACH slots. The FRACHPPDU may include the TXOP duration request.

In the example embodiment, MIMO frame transmissions use the MIMO PLCPHeader, detailed below. Since legacy 802.11b, 802.11a, and 802.11g STAsare able to decode only the SIGNAL1 field of the MIMO PLCP header(detailed with respect to FIG. 50, below), in the presence of non-MIMOSTAs, MIMO frames must be transmitted with protection. When both legacyand MIMO STAs are present, STAs using EDCA access procedures may use alegacy RTS/CTS sequence for protection. Legacy RTS/CTS refers to thetransmission of RTS/CTS frames using legacy preamble, PLCP header andMAC frame formats.

MIMO transmissions may also utilize the protection mechanisms providedby the 802.11e HCCA. Thus, transmissions from the AP to STAs, polledtransmissions from STAs to the AP, or from a STA to another STA (usingthe Direct Link Protocol) may be provided protection using theControlled Access Period (CAP).

The AP may also use legacy CTS-to-Self for protection of the MIMOScheduled Access Period (SCAP) from legacy STAs.

When an AP determines that all STAs present in the BSS are capable ofdecoding the MIMO PLCP header, it indicates this in a MIMO capabilitieselement in the Beacon. This is referred to as a MIMO BSS.

In a MIMO BSS, under both EDCA and HCCA, frame transmissions use theMIMO PLCP header and MIMO OFDM Training symbols according to MIMO OFDMTraining symbols aging rules. Transmissions in the MIMO BSS use the MIMOPLCP.

Reduced Inter-Frame Spacing

Various techniques for generally reducing Inter-Frame Spacing aredetailed above. Illustrated here are several examples of reducinginter-frame spacing in this example embodiment. For scheduledtransmissions, the start time of the TXOP is indicated in the SCHEDmessage. The transmitting STA may begin its scheduled TXOP at theprecise start time indicated in the SCHED message without determiningthat the medium is idle. As described above, consecutive scheduled APtransmissions during a SCAP are transmitted with no minimum IFS.

In the example embodiment, consecutive scheduled STA transmissions (fromdifferent STAs) are transmitted with an IFS of at least Guard IFS(GIFS). The default value of GIFS is 800 ns. A larger value may bechosen up to the value of Burst IFS (BIFS) defined next. The value ofGIFS may be indicated in the ACF capabilities element, described above.Alternate embodiments may employ any values for GIFS and BIFS.

Consecutive MIMO OFDM PPDU transmissions from the same STA (TXOPbursting) are separated by a BIFS. When operating in the 2.4 GHz band,the BIFS is equal to the 10 μs and the MIMO OFDM PPDU does not includethe 6 μs OFDM signal extension. When operating in the 5 GHz band, theBIFS is 10 μs. In an alternate embodiment, BIFS may be set to a smalleror larger value, including 0. To allow the receiving STA Automatic GainControl (AGC) to switch between transmissions, a gap larger than 0 maybe used when the transmitting STA transmit power is changed.

Frames that require an immediate response from the receiving STA are nottransmitted using a MIMO OFDM PPDU. Instead, they are transmitted usingthe underlying legacy PPDU, i.e. Clause 19 in the 2.4 GHz band or Clause17 in the 5 GHz band. Some examples of how legacy and MIMO OFDM PPDUsare multiplexed on the medium are shown below.

First, consider a legacy RTS/CTS followed by MIMO OFDM PPDU bursting.The transmission sequence is as follows: Legacy RTS-SIFS-LegacyCTS-SIFS-MIMO OFDM PPDU-BIFS-MIMO OFDM PPDU. In 2.4 GHz, the legacy RTSor CTS PPDU uses OFDM signal extension and the SIFS is 10 μs. In 5 GHz,there is no OFDM extension but the SIFS is 16 μs.

Second, consider an EDCA TXOP using MIMO OFDM PPDU. The transmissionsequence is as follows: MIMO OFDM PPDU-BIFS-LegacyBlockAckRequest-SIFS-ACK. The EDCA TXOP is obtained using EDCAprocedures for the appropriate Access Class (AC). As detailed above,EDCA defines access classes that may use different parameters per AC,such as AIFS[AC], CWmin[AC], and CWmax[AC]. The Legacy BlockAckRequestis transmitted with either signal extension or 16 μs SIFS. If theBlockAckRequest is transmitted in the aggregate frame within the MIMOOFDM PPDU, there is no ACK.

Third, consider consecutive scheduled TXOPs. The transmission sequenceis as follows: STA A MIMO OFDM PPDU-GIFS-STA B MIMO OFDM PPDU. There maybe an idle period after the transmission of the STA A MIMO OFDM PPDU ifthe PPDU transmission is shorter than the assigned maximum permittedTXOP time.

As described above, decoding and demodulation of coded OFDMtransmissions imposes additional processing requirements at thereceiving STA. To accommodate this, 802.11a and 802.11g allow additionaltime for the receiving STA before the ACK must be transmitted. In802.11a, the SIFS time is set to 16 μs. In 802.11g the SIFS time is setto 10 μs but an additional 6 μs OFDM signal extension is introduced.

Since decoding and demodulation of MIMO OFDM transmissions may imposeeven more processing burden, following the same logic, an embodiment maybe designed to increase the SIFS or OFDM signal extension, leading tofurther reduction in efficiency. In the example embodiment, by extendingthe Block ACK and Delayed Block Ack mechanisms of 802.11e, therequirement of immediate ACK for all MIMO OFDM transmissions iseliminated. Instead of increasing the SIFS or the signal extension, thesignal extension is eliminated, and for many situations the requiredinter-frame spacing between consecutive transmissions is reduced oreliminated, leading to greater efficiency.

SCHED Message

FIG. 45 illustrates the SCHED message, introduced above with respect toFIG. 41, and detailed further below. The SCHED message 4120 is amultiple poll message that assigns one or more AP-STA, STA-AP andSTA-STA TXOPs for the duration of a Scheduled Access Period (SCAP). Useof the SCHED message permits reduced polling and contention overhead, aswell as eliminates unnecessary IFS.

The SCHED message 4120 defines the schedule for the SCAP. SCHED message4120 comprises a MAC Header 4510 (15 octets in the example embodiment).In the example embodiment, each of the CTRL0, CTRL1, CTRL2 and CTRL3segments (referred to generically herein as CTRLJ, where J may be 0 to 3to illustrate segments 4515-4530, respectively) are of variable lengthand may be transmitted at 6, 12, 18 and 24 Mbps, respectively, whenpresent.

The example MAC header 4510 comprises Frame Control 4535 (2 octets),Duration 4540 (2 octets), BSSID 4545 (6 octets), Power Management 4550(2 octets), and MAP 4555 (3 octets). Bits 13-0 of the Duration field4540 specify the length of the SCAP in microseconds. The Duration field4540 is used by STAs capable of MIMO OFDM transmissions to set the NAVfor the duration of the SCAP. When legacy STAs are present in the BSS,the AP may use other means to protect the SCAP, e.g. a legacyCTS-to-Self. In the example embodiment, the maximum value of the SCAP is4 ms. The BSSID field 4545 identifies the AP.

The Power Management field 4550 is shown in FIG. 46. Power Management4550 comprises SCHED Count 4610, a reserved field 4620 (2 bits),Transmit Power 4630, and Receive Power 4640. The AP transmit power andAP receive power are as indicated in the Power Management field and STAreceive power level is measured at the STA.

SCHED Count is a field that is incremented at each SCHED transmission (6bits in this example). The SCHED Count is reset at each Beacontransmission. SCHED Count may be used for various purposes. As anexample, a power-saving feature using SCHED Count is described below.

The Transmit Power field 4630 represents the transmit power level beingused by the AP. In the example embodiment, the 4-bit field is encoded asfollows: The value represents the number of 4 dB steps that the transmitpower level is below the Maximum Transmit Power Level (in dBm) for thatchannel as indicated in an information element of the Beacon.

The Receive Power field 4640 represents the receive power level expectedat the AP. In the example embodiment, the 4-bit field is encoded asfollows: The value represents the number of 4 dB steps that the receivepower level is above the minimum Receiver Sensitivity Level (−82 dBm).Based on the received power level at a STA, a STA may compute itstransmit power level as follows: STA Transmit Power (dBm)=AP TransmitPower (dBm)+AP Receive Power (dBm)−STA Receive Power (dBm).

In the example embodiment, during scheduled STA-STA transmissions, thecontrol segment is transmitted at a power level that may be decoded atboth the AP as well as the receiving STA. A power control report fromthe AP or the Power Management field 4550 in the SCHED frame permits theSTA to determine the transmit power level required so that the controlsegment may be decoded at the AP. This general aspect is detailed abovewith respect to FIG. 22. For a scheduled STA-STA transmission, when thepower required to decode at the AP is different than the power requiredto decode at the receiving STA, the PPDU is transmitted at the higher ofthe two power levels.

The MAP field 4555, shown in FIG. 47, specifies the presence andduration of protected contention based access periods during the SCAP.MAP field 4555 comprises FRACH Count 4710, FRACH Offset 4720, and EDCAOffset 4730. The example FRACH Count 4710 (4 bits) is the number ofFRACH slots scheduled starting at the FRACH Offset 4720 (10 bits). EachFRACH slot is 28 ns. An FRACH Count value of ‘0’ indicates that there isno FRACH period in the current Scheduled Access Period. The EDCA Offset4730 is the start of the protected EDCA period. The example EDCA Offset4730 is 10 bits. Both the FRACH Offset 4720 and the EDCA Offset 4730 arein units of 4 μs starting from the beginning of the SCHED frametransmission.

The SCHED message 4120 is transmitted as a special SCHED PPDU 5100 (Type0010), detailed further below with respect to FIG. 51. The presencewithin SCHED message 4120 and length of the CTRL0 4515, CTRL1 4520,CTRL2 4525, and CTRL3 4530 segments are indicated in the SIGNAL field(5120 and 5140) of the PLCP Header of the SCHED PPDU 5100.

FIG. 48 illustrates SCHED control frames for TXOP assignment. Each ofthe CTRL0 4515, CTRL1 4520, CTRL2 4525, and CTRL3 4530 segments are ofvariable length and each comprises zero or more assignment elements(4820, 4840, 4860, and 4880, respectively). A 16-bit FCS (4830, 4850,4870, and 4890, respectively) and 6 tail bits (not shown) are added perCTRLJ segment. For the CTRL0 segment 4515 the FCS is computed over theMAC Header 4510 and any CTRL0 assignment elements 4820 (thus MAC Headeris shown prepended to CTRL0 4515 in FIG. 48). In the example embodiment,the FCS 4830 for CTRL0 4515 is included even if no assignment elementsare included in the CTRL0 segment.

As detailed herein, the AP transmits assignments for AP-STA, STA-AP andSTA-STA transmissions in the SCHED frame. Assignment elements todifferent STAs are transmitted in a CTRLJ segment as indicated by theSTA in the SCHED Rate field of the PLCP header of its transmissions.Note that CTRL0 through CTRL3 correspond to decreasing robustness. EachSTA begins decoding the PLCP Header of the SCHED PPDU. The SIGNAL fieldindicates the presence and length of CTRL0, CTRL1, CTRL2 and CTRL3segments in the SCHED PPDU. The STA receiver begins with decoding theMAC Header and CTRL0 segment, decoding each assignment element until theFCS, and it continues to subsequently decode CTRL1, CTRL2 and CTRL3,stopping at the CTRLJ segment whose FCS it is unable to verify.

Five types of assignment elements are defined as shown in Table 3. Anumber of assignment elements may be packed into each CTRLJ segment.Each assignment element specifies the transmitting STA Access ID (AID),the receiving STA AID, the start time of the scheduled TXOP and themaximum permitted length of the scheduled TXOP.

TABLE 3 Assignment Element Types Total Type Assignment Fields Length (3bits) Element Type (Lengths in bits) in bits 000 Simplex AP-STA PreamblePresent  (1) 40 AID (16) Start Offset (10) TXOP Duration (10) 001Simplex STA-AP AID (16) 39 Start Offset (10) TXOP Duration (10) 010Duplex AP-STA Preamble Present  (1) 60 AID (16) AP Start Offset (10) APTXOP Duration (10) STA Start Offset (10) STA TXOP Duration (10) 011Simplex STA-STA Transmit AID (16) 55 Receive AID (16) Start Offset (10)Max PPDU Size (10) 100 Duplex STA-STA AID 1 (16) 75 AID 2 (16) STA 1Start Offset (10) STA 1 Max PPDU Size (10) STA 2 Start Offset (10) STA 2Max PPDU Size (10)

The preamble may be eliminated in consecutive transmissions from the AP.The Preamble Present bit is set to 0 if the AP will not transmit apreamble for a scheduled AP transmission. An example benefit of preambleelimination is when the AP has low bandwidth, low latency flows toseveral STAs, such as in a BSS with many Voice over IP (VoIP) flows.Therefore, the SCHED frame permits the aggregation of transmissions fromthe AP to several receiving STAs (i.e. PPDU aggregation, describedabove). Frame Aggregation, as defined above, permits the aggregation offrames to one receiving STA.

The Start Offset field is in multiples of 4 μs referenced from the starttime of the SCHED message preamble. The AID is the Access ID of theassigned STA(s).

For all assignment element types except scheduled STA-STA transmissions,the TXOP Duration field is the maximum permitted length of the scheduledTXOP in multiples of 4 μs. The actual PPDU Size of the transmitted PPDUis indicated in the SIGNAL1 field of the PPDU (detailed further below).

For scheduled STA-STA transmissions (Assignment Element Types 011 and100), the Max PPDU Size field is also the maximum permitted length ofthe scheduled TXOP in multiples of 4 μs, however additional rules mayapply. In the example embodiment, for scheduled STA-STA transmissions,the TXOP contains only one PPDU. The receiving STA uses the Max PPDUSize indicated in the assignment element to determine the number of OFDMsymbols in the PPDU (since the PPDU Size field is replaced by a Requestfield in the SIGNAL1, detailed below with respect to FIG. 51). If theSTA-STA flow uses OFDM symbols with the standard Guard Interval (GI),the receiving STA sets the PPDU Size for the scheduled TXOP to the MaxPPDU Size indicated in the assignment element. If the STA-STA flow usesOFDM symbols with shortened GI, the receiving STA determines the PPDUSize by scaling up the Max PPDU Size field by a factor of 10/9 androunding down. The transmitting STA may transmit a PPDU shorter than theassigned Max PPDU Size. The PPDU Size does not provide the length of theaggregated MAC frame to the receiver. The length of the encapsulatedframes is included in the Aggregation header of each MAC frame.

Inclusion of the transmitting and receiving STA in the assignmentelements permits power saving at STAs that are not scheduled to transmitor receive during the SCAP. Recall the SCHED Count field introducedabove. Each assignment scheduled by the SCHED message specifies thetransmitting STA AID, the receiving STA AID, the start time of thescheduled TXOP, and the maximum permitted length of the scheduled TXOP.The SCHED Count is incremented at each SCHED transmission and is resetat each Beacon transmission. STAs may indicate a power-save operation tothe AP, and thus are provided specific SCHED Count values during whichthey may be assigned scheduled transmit or receive TXOPs by the AP. STAsmay then wake up periodically only to listen for SCHED messages with anappropriate SCHED Count.

PPDU Formats

FIG. 49 depicts a legacy 802.11 PPDU 4970, comprising a PLCP preamble4975 (12 OFSM symbols), a PLCP header 4910, a variable length PSDU 4945,a 6-bit tail 4950, and variable length pad 4955. A portion 4960 of PPDU4970 comprises a SIGNAL field (1 OFDM symbol) transmitted using BPSK atrate=1/2, and a variable length data field 4985, transmitted with themodulation format and rate indicated in SIGNAL 4980. PLCP header 4910comprises SIGNAL 4980 and 16-bit Service field 4940 (which is includedin DATA 4985, and transmitted according to its format). SIGNAL field4980 comprises Rate 4915 (4 bits), reserved field 4920 (1 bit), Length4925 (12 bits), Parity bit 4930, and Tail 4935 (6 bits).

The extended SIGNAL fields (detailed below) in the example PLCP Header(detailed below) is backward compatible with the SIGNAL field 4980 oflegacy 802.11. Unused values of the RATE field 4915 in legacy SIGNALfield 4980 are set to define new PPDU types (detailed below).

Several new PPDU types are introduced. For backward compatibility withlegacy STAs, the RATE field in the SIGNAL field of the PLCP Header ismodified to a RATE/Type field. Unused values of RATE are designated asPPDU Type. The PPDU Type also indicates the presence and length of aSIGNAL field extension designated SIGNAL2. New values of the RATE/Typefield are defined in Table 4. These values of the RATE/Type field areundefined for legacy STAs. Therefore, legacy STAs will abandon decodingof the PPDU after successfully decoding the SIGNAL1 field and finding anundefined value in the RATE field.

Alternately, the Reserved bit in the legacy SIGNAL field may be set to‘1’ to indicate a MIMO OFDM transmission to a new class STA. ReceivingSTAs may ignore the Reserved bit and continue to attempt to decode theSIGNAL field and the remaining transmission.

The receiver is able to determine the length of the SIGNAL2 field basedon the PPDU Type. The FRACH PPDU appears only in a designated portion ofthe SCAP and needs to be decoded only by the AP.

TABLE 4 MIMO PPDU Types SIGNAL2 Field RATE/Type Length (OFDM (4 bits)MIMO PPDU Symbols) 0000 MIMO BSS IBSS or MIMO 1 AP transmission (exceptSCHED PPDU). 0010 MIMO BSS SCHED PPDU 1 0100 MIMO BSS FRACH PPDU 2

FIG. 50 depicts MIMO PPDU format 5000 for data transmissions. PPDU 5000is referred to as PPDU Type 0000. PPDU 5000 comprises a PLCP preamble5010, SIGNAL 1 5020 (1 OFDM symbol), SIGNAL 2 5040 (1 OFDM symbol),Training Symbols 5060 (0, 2, 3, or 4 symbols), and a variable lengthData field 5080. PLCP preamble 5010, when present, is 16 μs in theexample embodiment. SIGNAL 1 5020 and SIGNAL 2 5040 are transmittedusing the PPDU control segment rate and modulation format. Data 5080comprises Service 5082 (16 bits), Feedback 5084 (16 bits), a variablelength PSDU 5086, Tail 5088 (6 bits per stream) where a separateconvoutional channel code is applied to each stream, and variable lengthPad 5090. Data 5080 is transmitted using the PPDU data segment rate andmodulation format.

The MIMO PLCP header for PPDU Type 0000 comprises the SIGNAL (includingSIGNAL1 5020 and SIGNAL2 5040), SERVICE 5082 and FEEDBACK 5084 fields.The SERVICE field is unchanged from legacy 802.11, and is transmittedusing the data segment rate and format.

The FEEDBACK field 5084 is transmitted using the data segment rate andformat. The FEEDBACK field comprises the ES field (1 bit), the Data RateVector Feedback (DRVF) field (13 bits), and a Power Control field (2bits).

The ES field indicates the preferred steering method. In the exampleembodiment, Eigenvector Steering (ES) is selected when the ES bit isset, and Spatial Spreading (SS) is selected otherwise.

The Data Rate Vector Feedback (DRVF) field provides feedback to the peerstation regarding the sustainable rate on each of up to four spatialmodes.

Explicit rate feedback allows stations to quickly and accuratelymaximize their transmission rates, dramatically improving efficiency ofthe system. Low latency feedback is desirable. However, feedbackopportunities need not be synchronous. Transmission opportunities may beobtained in any manner, such as contention-based (i.e. EDCA), polled(i.e. HCF), or scheduled (i.e. ACF). Therefore, variable amounts of timemay pass between transmission opportunities and rate feedback. Based onthe age of the rate feedback, the transmitter may apply a back-off todetermine the transmission rate.

The PPDU data segment rate adaptation for transmissions from STA A toSTA B relies on feedback provided by STA B to STA A (described earlier,see FIG. 24, for example). For either ES or SS mode of operation, eachtime STA B receives MIMO OFDM Training Symbols from the STA A, itestimates the data rates that can be achieved on each spatial stream. Inany subsequent transmission from STA B to STA A, STA B includes thisestimate in the DRVF field of FEEDBACK 5084. The DRVF field istransmitted at the data segment 5080 rate.

When transmitting to STA B, STA A determines what transmission rates touse based on the DRVF it received from STA B, with an optional back-offas necessary to account for delays. The SIGNAL field (detailed below)contains the 13-bit DRV field 5046 that allows the receiving STA B todecode the frame transmitted from STA A. The DRV 5046 is transmitted atthe control segment rate.

The DRVF field is encoded comprising a STR field (4 bits), an R2 field(3 bits), an R3 field (3 bits), and an R4 field (3 bits). The STR fieldindicates the Rate for Stream 1. This field is coded as STR Value shownin Table 5. R2 indicates the difference between the STR Value for Stream1 and the STR Value for Stream 2. An R2 value of “111” indicates thatStream 2 is off R3 indicates the difference between the STR Value forStream 2 and the STR Value for Stream 3. An R3 value of “111” indicatesthat Stream 3 is off. If R2=“111”, then R3 is set to “111”. R4 indicatesthe difference between the STR Value for Stream 3 and the STR Value forStream 4. An R4 value of “111” indicates that Stream 4 is off. IfR3=“111” then R4 is set to “111”.

When ES=0, i.e. spatial spreading, an alternate encoding of the DRVF isas follows: Number of Streams (2 bits), Rate per Stream (4 bits). TheRate per Stream field is coded as STR Value above. The remaining 7 bitsare Reserved.

TABLE 5 STR Encoding Bits/symbol STR Value Coding Rate Modulation Formatper Stream 0000 1/2 BPSK 0.5 0001 3/4 BPSK 0.75 0010 1/2 QPSK 1.0 00113/4 QPSK 1.5 0100 1/2 16 QAM 2.0 0101 5/8 16 QAM 2.5 0110 3/4 16 QAM 3.00111  7/12 64 QAM 3.5 1000 2/3 64 QAM 4.0 1001 3/4 64 QAM 4.5 1010 5/664 QAM 5.0 1011 5/8 256 QAM 5.0 1100 3/4 256 QAM 6.0 1101 7/8 256 QAM7.0

In addition to the DRVF, STA B also provides power control feedback tothe transmitting STA A. This feedback is included in the Power Controlfield and is also transmitted at the data segment rate. This field is 2bits and indicates either to increase or decrease power or to leave thepower level unchanged. The resultant transmit power level is designatedthe Data Segment Transmit Power level.

Example Power Control field values are illustrated in Table 6. Alternateembodiments may deploy power control fields of various sizes, and withalternate power adjustment values.

TABLE 6 Power Control Field Values Power Control Field Meaning 00 NoChange 01 Increase power by 1 dB 10 Decrease power by 1 dB 11 Reserved

The transmit power level remains constant for the entire PPDU. When theData Segment Transmit Power Level and the Open Loop STA Transmit Power(i.e. the power level required for the AP to decode the transmission,detailed above) are different, the PPDU is transmitted at the maximum ofthe two power levels. That is, PPDU Transmit Power Level is the maximumof the Open Loop STA Transmit Power (dBm) and the Data Segment TransmitPower (dBm).

In the example embodiment, the Power Control field is set to “00” in thefirst frame of any frame exchange sequence. In subsequent frames, itindicates the increase or decrease of power in 1 dB steps. The receivingSTA will use this feedback information in all subsequent frametransmissions to that STA.

SIGNAL1 5020 comprises RATE/Type field 5022 (4 bits), 1 Reserved Bit5024, PPDU Size/Request 5026 (12 bits), Parity bit 5028, and a 6-bitTail 5030. The SIGNAL1 field 5020 is transmitted using the controlsegment rate and format (6 Mbit/s, in the example embodiment). TheRATE/Type field 5022 is set to 0000. The Reserved bit 5024 may be set to0.

The PPDU Size/Request Field 5026 serves two functions, depending on thetransmission mode. In contention-based STA transmissions and all APtransmissions, this field denotes the PPDU Size. In this first mode, Bit1 indicates that the PPDU uses expanded OFDM symbols, Bit 2 indicatesthat the PPDU uses OFDM symbols with shortened GI, and Bits 3-12indicate the number of OFDM symbols.

In scheduled non-AP STA transmissions, PPDU Size/Request Field 5026denotes Request. In this second mode, Bits 1-2 indicate the SCHED Rate.SCHED Rate indicates the highest numbered SCHED (0, 1, 2 or 3) fieldthat may be used to transmit an assignment to the STA. During Trainingsymbol transmissions from the AP, each non-AP STA estimates the rate atwhich it can robustly receive SCHED frame transmissions from the AP. Insubsequent scheduled transmissions from the STA, this maximumpermissible rate is included in the SCHED Rate field. This field isdecoded by the AP. The AP uses this information to schedule subsequentTXOPs for the STA and determines the CTRLJ (0, 1, 2, or 3) for issuingthose allocations to the STA.

In the second mode, Bits 3-4 indicate the QoS field, which identifiesthe fraction (in thirds) of the request that is for TC 0 or 1 (i.e. 0%,33%, 67%, 100%). Bits 5-12 indicate the requested length of TXOP (inmultiples of 16 μs, in the example embodiment).

The SIGNAL1 field 5020 is checked by 1 Parity bit 5028 and terminatedwith a 6-bit Tail 5030 for the convolutional encoder.

The presence and length of the SIGNAL2 field 5040 is indicated by theRATE/Type field 5022 in SIGNAL1 5020. The SIGNAL2 field 5040 istransmitted using the control segment rate and format. SIGNAL2 5040comprises a Reserved bit 5042, Training Type 5044 (3 bits), Data RateVector (DRV) 5046 (13 bits), Parity bit 5048, and Tail 5050 (6 bits).The 3-bit Training Type field indicates the length and format of theMIMO OFDM Training symbols. Bits 1-2 indicate the number of MIMO OFDMTraining Symbols 5060 (0, 2, 3 or 4 OFDM symbols). Bit 3 is the TrainingType field: 0 indicates SS, 1 indicates ES. The DRV 5046 provides therate for each of up to four spatial modes. The DRV 5046 is encoded inthe same manner as DRVF (included in FEEDBACK 5084, detailed above). TheSIGNAL2 field 5040 is checked by 1 Parity bit 5048 and terminated with a6-bit Tail 5050 for the convolutional encoder.

FIG. 51 depicts SCHED PPDU 5100 (Rate/Type=0010). SCHED PPDU 5100comprises a PLCP preamble 5110, SIGNAL 1 5120 (1 OFDM symbol), SIGNAL 25140 (1 OFDM symbol), Training Symbols 5160 (0, 2, 3, or 4 symbols), anda variable length SCHED Frame 5180. PLCP preamble 5010, when present, is16 μs in the example embodiment. SIGNAL 1 5020 and SIGNAL 2 5040 aretransmitted using the PPDU control segment rate and modulation format.SCHED Frame 5180 may include various rates, as detailed above, withrespect to the ACF description.

SIGNAL1 5120 comprises RATE/Type 5122 (4 bits), a Reserved bit 5124,CTRL0 Size 5126 (6 bits), CTRL1 Size 5128 (6 bits), Parity bit 5130, andTail 5132 (6 bits). RATE/Type 5122 is set to 0010. The Reserved bit 5124may be set to 0. CTRL0 Size 5126 indicates the length of the segment ofthe SCHED PPDU transmitted at the lowest rate (6 Mbps in this example).This segment includes the SERVICE field of the PLCP Header, the MACHeader and the CTRL0 segment 5126. The value is encoded in multiples of4 μs, in this example. CTRL1 Size 5128 indicates the length of thesegment of the SCHED PPDU transmitted at the next higher rate (12 Mbpsin this example). The value is encoded in multiples of 4 μs, in thisexample. A CTRL1 Size of ‘0’ indicates that the corresponding CTRL1segment is not present in the SCHED PPDU. The SIGNAL1 field 5120 ischecked by 1 Parity bit 5130 and terminated with a 6-bit Tail 5132 forthe convolutional encoder.

SIGNAL2 5140 comprises a Reserved bit 5142, Training Type 5144 (3 bits),CTRL2 Size 5146 (5 bits), CTRL3 Size 5148 (5 bits), FCS 5150 (4 bits),and Tail 5152 (6 bits). The Reserved bit 5142 may be set to 0. TrainingType 5144 is as specified for PPDU Type 0000 (Training Type 5044).

CTRL2 Size 5146 indicates the length of the segment of the SCHED PPDUtransmitted at the next highest rate (18 Mbps in this example). Thevalue is encoded in multiples of 4 μs, in this example. A CTRL2 Size of‘0’ indicates that the corresponding CTRL2 segment is not present in theSCHED PPDU. CTRL3 Size 5148 indicates the length of the segment of theSCHED PPDU transmitted at the highest rate (24 Mbps in this example).The value is encoded in multiples of 4 μs, in this example. A CTRL2 Sizeof ‘0’ indicates that the corresponding CTRL3 segment is not present inthe SCHED PPDU.

FCS 5150 is computed over the entire SIGNAL1 and SIGNAL2 fields. TheSIGNAL2 field 5152 is terminated with a 6-bit Tail 5152 for theconvolutional encoder.

FIG. 52 depicts FRACH PPDU 5200 (Rate/Type=0100). FRACH PPDU 5200comprises a PLCP preamble 5210, SIGNAL 1 5220 (1 OFDM symbol), andSIGNAL 2 5240 (2 OFDM symbols). PLCP preamble 5210, when present, is 16μs in the example embodiment. SIGNAL 1 5220 and SIGNAL 2 5240 aretransmitted using the PPDU control segment rate and modulation format.The FRACH PPDU 5200 is transmitted by a STA during the FRACH periodwithin the MIMO Scheduled Access Period. The FRACH period is establishedby and therefore known to the AP (as detailed above).

SIGNAL1 5220 comprises RATE/Type 5222 (4 bits), a Reserved bit 5224,Request 5226 (12 bits), Parity bit 5228, and Tail 5230 (6 bits).RATE/Type 5222 is set to 0100. The Reserved bit 5124 may be set to 0.The Request Field 5226 is as specified for PPDU Type 0000 (5000),detailed above. The SIGNAL1 field 5220 is checked by 1 Parity bit 5228and terminated with a 6-bit Tail 5230 for the convolutional encoder.

SIGNAL2 5240 comprises a Reserved bit 5242, Source AID 5244 (16 bits),Destination AID 5246 (16 bits), FCS 5248 (4 bits), and Tail 5250 (6bits). The Reserved bit 5242 may be set to 0. Source AID 5244 identifiesthe STA transmitting on the FRACH. Destination AID 5246 identifies thedestination STA for which a TXOP is being requested. In the exampleembodiment, in the case where the destination is the AP, the value ofthe Destination AID field 5246 is set to 2048. A 4-bit FCS 5248 iscomputed over the entire SIGNAL1 and SIGNAL2 fields. A 6 bit Tail 5250is added prior to convolutional encoding.

In the example embodiment, STAs may use slotted Aloha to access thechannel and transmit the request message in the FRACH. If receivedsuccessfully by the AP, the AP provides the requesting STA with ascheduled TXOP in a subsequent scheduled access period. The number ofFRACH slots for the current scheduled access period is indicated in theSCHED message, N_FRACH.

The STA may also maintain a variable B_FRACH. Following a transmissionon the FRACH, if the STA receives a TXOP assignment from the AP, itresets BFRACH. If the STA does not receive a TXOP assignment within apredetermined number, FRACH RESPONSE, of SCHED transmissions from theAP, B_FRACH is incremented by 1 up to a maximum value of 7. Theparameter FRACH RESPONSE is included in an ACF element of the Beacon.During any FRACH, the STA picks a FRACH slot with probability(N_FRACH)⁻¹*2^(−B) ^(—) ^(FRACH).

If no FRACH period is scheduled by the AP, MIMO STAs may contend duringthe protected contention period during the SCAP using EDCA rules.

Those of skill in the art would understand that information and signalsmay be represented using any of a variety of different technologies andtechniques. For example, data, instructions, commands, information,signals, bits, symbols, and chips that may be referenced throughout theabove description may be represented by voltages, currents,electromagnetic waves, magnetic fields or particles, optical fields orparticles, or any combination thereof.

Those of skill would further appreciate that the various illustrativelogical blocks, modules, circuits, and algorithm steps described inconnection with the embodiments disclosed herein may be implemented aselectronic hardware, computer software, or combinations of both. Toclearly illustrate this interchangeability of hardware and software,various illustrative components, blocks, modules, circuits, and stepshave been described above generally in terms of their functionality.Whether such functionality is implemented as hardware or softwaredepends upon the particular application and design constraints imposedon the overall system. Skilled artisans may implement the describedfunctionality in varying ways for each particular application, but suchimplementation decisions should not be interpreted as causing adeparture from the scope of the present invention.

The various illustrative logical blocks, modules, and circuits describedin connection with the embodiments disclosed herein may be implementedor performed with a general purpose processor, a digital signalprocessor (DSP), an application specific integrated circuit (ASIC), afield programmable gate array (FPGA) or other programmable logic device,discrete gate or transistor logic, discrete hardware components, or anycombination thereof designed to perform the functions described herein.A general purpose processor may be a microprocessor, but in thealternative, the processor may be any conventional processor,controller, microcontroller, or state machine. A processor may also beimplemented as a combination of computing devices, e.g., a combinationof a DSP and a microprocessor, a plurality of microprocessors, one ormore microprocessors in conjunction with a DSP core, or any other suchconfiguration.

The steps of a method or algorithm described in connection with theembodiments disclosed herein may be embodied directly in hardware, in asoftware module executed by a processor, or in a combination of the two.A software module may reside in RAM memory, flash memory, ROM memory,EPROM memory, EEPROM memory, registers, hard disk, a removable disk, aCD-ROM, or any other form of storage medium known in the art. Anexemplary storage medium is coupled to the processor such the processorcan read information from, and write information to, the storage medium.In the alternative, the storage medium may be integral to the processor.The processor and the storage medium may reside in an ASIC. The ASIC mayreside in a user terminal. In the alternative, the processor and thestorage medium may reside as discrete components in a user terminal.

Headings are included herein for reference and to aid in locatingvarious sections. These headings are not intended to limit the scope ofthe concepts described with respect thereto. Such concepts may haveapplicability throughout the entire specification.

The previous description of the disclosed embodiments is provided toenable any person skilled in the art to make or use the presentinvention. Various modifications to these embodiments will be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other embodiments without departing from thespirit or scope of the invention. Thus, the present invention is notintended to be limited to the embodiments shown herein but is to beaccorded the widest scope consistent with the principles and novelfeatures disclosed herein.

What is claimed is:
 1. A method of data transmission by a new classaccess point (AP), comprising: receiving a beacon from a legacy accesspoint (AP) indicating a prescribed contention period; and contending foraccess to a shared medium for a duration of the prescribed contentionperiod, wherein at least one new class station (STA) accesses the sharedmedium during the duration.
 2. The method of claim 1, wherein the beaconidentifies a legacy contention free period and a contention period, andfurther comprising: refraining from contending for access during thelegacy contention free period indicated in the beacon.
 3. The method ofclaim 2, wherein legacy contention free polls are carried out during thelegacy contention free period.
 4. The method of claim 2, wherein STAscontend for access to the shared medium during the contention freeperiod.
 5. The method of claim 1, wherein the at least one new class STAis in a basic service set (BSS) of the new class AP.
 6. The method ofclaim 1, wherein the new class AP and the legacy AP share a singlefrequency assignment (FA).
 7. The method of claim 1, further comprisingtransmitting a legacy signal to at least one legacy STA indicating theperiod.
 8. The method of claim 1, wherein the at least one legacy STArefrains from accessing the shared medium during the period.
 9. Themethod of claim 1, wherein the period comprises one or more timedivision multiplexing (TDD) medium access control (MAC) frame intervals.10. The method of claim 1, wherein the contending is performed at timedintervals.
 11. The method of claim 1, further comprises activating atimer at the beginning of the period set to expire at the end of theperiod.
 12. The method of claim 1, further comprising synching to atarget beacon transmission time (TBTT) associated with the beacon.
 13. Amethod of data transmission by a user terminal, comprising: contendingfor access to a shared medium; clearing the shared medium using a legacysignal; transmitting a request to a second user terminal; receiving aresponse from the second user terminal, wherein the response compriseschannel feedback from the second user terminal based on the transmittedrequest; and transmitting a steered pilot and steered data to the seconduser terminal based on the feedback wherein the transmitting and thereceiving are over the shared medium.
 14. The method of claim 13,further comprising transmitting a pilot along with the request to thesecond user terminal.
 15. The method of claim 13, wherein the channelfeedback comprises rate feedback.
 16. The method of claim 13, whereinthe response further comprises an acknowledgment (ACK).
 17. The methodof claim 13, wherein the first user terminal and the second userterminal are new class user terminals.
 18. The method of claim 13,wherein the response further comprises the basic service set (BSS) ID ofthe second user terminal.
 19. The method of claim 13, wherein the seconduser terminal is in a different basic service set (BSS) than the userterminal.
 20. The method of claim 13, wherein the response furthercomprises a steered pilot.
 21. The method of claim 20, wherein the pilotis received within a short interframe spacing (SIFS) in response to therequest.
 22. The method of claim 13, wherein the clearing comprisesbroadcasting at least one of a clear-to-send message or a ready-to-sendmessage.
 23. The method of claim 22, wherein the at least one messageincludes the user terminal's basic service set (BSS) ID, medium accesscontrol (MAC) ID, and the second user terminal's MAC ID.
 24. A programproduct comprising a non-transitory computer readable medium havinginstructions stored thereon, the instructions generally executable byone or more processors for data transmission by a station (STA), theinstructions comprising: code for contending for access to a sharedmedium; code for clearing the shared medium using a legacy signal; codefor transmitting a request to a second STA; code for receiving aresponse from the second STA, wherein the response comprises channelfeedback from the second STA based on the transmitted request; and codefor transmitting a steered pilot and steered data to the second STAbased on the feedback, wherein the transmitting and the receiving areover the shared medium.
 25. A method of data transmission, comprising:transmitting a scheduling control message (SCHED) comprising a pluralityof transmission opportunities (TXOPs) for a plurality of remote stations(STAs), wherein the SCHED comprises a plurality of control segments, andwherein the plurality of control segments are encoded at a tieredencoding rate wherein different encoding rates are applied to differentcontrol segments; and exchanging frames of data with the plurality ofremote stations in accordance with the plurality of TXOPs in thescheduling control message.
 26. The method of claim 25, wherein theSCHED further comprises a medium access control (MAC) header.
 27. Themethod of claim 26, wherein the MAC header includes a duration field, abasic service set (BSS) identification (ID) field, a power managementfield, and a MAP field.
 28. The method of claim 25, wherein theplurality of control segments comprises: a first control segment encodedat 6 Mbps; a second control segment encoded at 12 Mbps; a third controlsegment encoded at 18 Mbps; and a fourth control segment encoded at 24Mbps.
 29. An apparatus for data transmission by a new class access point(AP), comprising: at least one processor configured to: receive a beaconfrom a legacy access point (AP) indicating a prescribed contentionperiod; and contend for access to a shared medium for a duration of theprescribed contention period, wherein at least one new class station(STA) accesses the shared medium during the duration; and a memorycoupled with the at least one processor.
 30. The apparatus of claim 29,wherein the beacon identifies a legacy contention free period and acontention period, and wherein the at least one processor is furtherconfigured to: refrain from contending for access during the legacycontention free period indicated in the beacon.
 31. The apparatus ofclaim 29, wherein the contending is performed at timed intervals. 32.The apparatus of claim 29, wherein the at least one processor is furtherconfigured to synch to a target beacon transmission time (TBTT)associated with the beacon.
 33. An apparatus for data transmission by auser terminal, comprising: at least one processor configured to: contendfor access to a shared medium; clear the shared medium using a legacysignal; transmit a request to a second user terminal; receive a responsefrom the second user terminal, wherein the response comprises channelfeedback from the second user terminal based on the transmitted request;and transmit a steered pilot and steered data to the second userterminal based on the feedback, wherein the transmitting and thereceiving are over the shared medium; and a memory coupled with the atleast one processor.
 34. The apparatus of claim 33, wherein the at leastone processor is further configured to transmit a pilot along with therequest to the second user terminal.
 35. The apparatus of claim 33,wherein the response further comprises an acknowledgment (ACK).
 36. Theapparatus of claim 33, wherein the response further comprises a basicservice set (BSS) ID of the second user terminal.
 37. The apparatus ofclaim 33, wherein the response further comprises a steered pilot.
 38. Aprogram product comprising a non-transitory computer readable mediumhaving instructions stored thereon, the instructions generallyexecutable by one or more processors for data transmission, theinstructions comprising: code for transmitting a scheduling controlmessage (SCHED) comprising a plurality of transmission opportunities(TXOPs) for a plurality of remote stations (STAs), wherein the SCHEDcomprises a plurality of control segments, and wherein the plurality ofcontrol segments are encoded at a tiered encoding rate wherein differentencoding rates are applied to different control segments; and code forexchanging frames of data with the plurality of remote stations inaccordance with the plurality of TXOPs in the scheduling controlmessage.
 39. An apparatus for data transmission, comprising: at leastone processor configured to: transmit a scheduling control message(SCHED) comprising a plurality of transmission opportunities (TXOPs) fora plurality of remote stations (STAs), wherein the SCHED comprises aplurality of control segments, and wherein the plurality of controlsegments are encoded at a tiered encoding rate wherein differentencoding rates are applied to different control segments; and exchangeframes of data with the plurality of remote stations in accordance withthe plurality of TXOPs in the scheduling control message; and a memorycoupled with the at least one processor.
 40. The apparatus of claim 39,wherein the plurality of control segments comprises: a first controlsegment encoded at 6 Mbps; a second control segment encoded at 12 Mbps;a third control segment encoded at 18 Mbps; and a fourth control segmentencoded at 24 Mbps.
 41. A program product comprising a non-transitorycomputer readable medium having instructions stored thereon, theinstructions generally executable by one or more processors for datatransmission by a new class access point (AP), the instructionscomprising: code for receiving a beacon from a legacy access point (AP)indicating a prescribed contention period; and code for contending foraccess to a shared medium for a duration of the prescribed contentionperiod, wherein at least one new class station (STA) accesses the sharedmedium during the duration.